Reactive Training Systems

I experience much the same thing when I do a lot of sets (such as on a Sheiko-cycle). The first set is hard, second even harder and the third is a pain in the ***. Then for some reason the weight moves a lot easier.

To determine whether or not it is beneficial, we need to look at possible causes of this phenomenon. Im not well versed in this particular circumstance, so if someone is aware of less-than-speculative research, let me know. From what I know of how the neuromuscular system works, this could be a result of several things.
There could be a neuromuscular reflex that increases efficiency with additional fatigue. I know wave loading proponents claim this. I have seen the effect-in-question myself, and anecdotally, I tend to think this has something to do with it, but I have nothing to support it. Additionally, it runs counter to logic of the neuromuscular system (at least the logic I am privy to anyway). In effect, I wouldnt be surprised if there was some sort of effect similar to this one, but I doubt it is a major player when faced with appreciable levels of fatigue.
Psychological. This explains a lot and encompasses a lot. Maybe you rested longer for the last set. Psychologically, we would tend to not notice a 1-2 minute increase in our rest periods when we are tired and worn down. This could, possibly, account for the improved performance. But maybe you track your rest periods closely. The next thing to account for in the realm of psychological perception is that maybe it *felt* easier because it was your last set and you were done, so your psychological perception was more positive. People who experience such effects would tend to not notice them, and subsequently discount claims that this accounts for their experiences (like Im making now). This could be disproved if one had Tendo data showing performance improvement in the latter sets, but barring the the presentation of that data, this possibility cant be outright denied.
Another psychological possibility is even more out there. You must also consider the perspective that, for whatever reason, you mentally put yourself into a higher gear for that set (aggressiveness returns for last set, personality differences, etc, etc, ad nauseum). Maybe you do actually perform better in the latter sets, but maybe thats because you will it to be.
So at this point, we can reasonably conclude that it is either a neuromuscular reflex that allows this to happen or it is psychological. Well, in either case, what is the benefit of achieving such an effect? Thats the question that must be answered. And the answer will be context-dependant. To me and those I train, I feel that in most circumstances, achieving a customized level of training volume that is honed in on their particular abilities is more beneficial than the possible training effects this phenomenon might induce. And regardless, especially in terms of the psychological preparation, there are other ways to achieve this effect.

I hope nobody takes this as me saying, "it's a figment of your imagination." It certainly isn't. But I do feel it's probably (probably) psychological in origin and that is very, very important. Thanks for the question! Intriguing!

Thanks for your response Mike. I have no doubt psychological influences play a role, I'm well aware of their effects across all areas of life. Your post has made me think a lot about this...

The thing is I really didn't need a tendo unit to determine that the last sets went better, the 3rd set was grinding really hard, we're talking seconds to reach lock out and an almost stall on the way, I even ripped my hand trying to hold onto the bar so long and had to tape it up! The last set was hard but they were pinging off the floor and making lockout with no stalling and I'm happy I could have achieved 5 reps.

If the effects were psychological, it may have something to do with more focus on the later sets. I usually get this after something has nearly beaten me, for example last time I went for maxes, I pulled a very hard 190k then 195 wouldn't budge off the floor. Rather than call it a day I got a bit wound up, decided to focus and at least get it off the floor. Then I locked it out, a grinder but I got there and this can only be due to a different mind set. On the bench too, I nearly always miss my second attempt by cramping my back at the bottom and nail the same weight on my third.

Now if only I could figure out a way to get to this level of focus without having to be either beaten or nearly beaten first...

Fuel for discussion anyway.

I feel that for some sports, the psychological/mental aspect is very overlooked. Personally i have noticed lots of similarities between wrestling and powerlifting, which on the surface are very physical, but also takes a large amount of mental strength as well. A very helpful tool is Ed Coans book which i would recommend for anyone. Ed is a big believer in mental visualization, so much so that he says hes visualized the entire meet so many times and thinks of how light the weight feels that it is impossible to fail because he has seen himself succeed so many times at it. Now again he is ed coan and is a legend, but i think these techniques can be used by anyone.

Me personally i put my ipod on, and i clear my mind right before a work set. i imagine all the times in my life that ive failed, whether its lifting, when i was in college, girlfriends, anything. i take all that energy and anger and channel it into physical power. i take all those instances and say to myself "you just try and stop me now". when i grab the bar i feel as though i could just rip it apart with my bare hands. this may not work for everyone, but it works for me. Ive helped training partners and friends with this as well.

One way to break out of a failure mindset is to take something positive away from each workout. you may have not hit a pr and feel that you have failed, but always look for something positive. for example my training partner was trying to hit a 1 rm on the bench. his best raw is 460. he warmed up and hit 405x1, then 440x1, missed 470, then dropped to 405x4 (his previous 5 rm is 405 and was his first workset). now even though he didnt set a pr, he handled 440 pretty easily which is 95% of his max, and then was only 1 rep short of his 5rm with 405. its important to always come away from a training session feeling good about something.

Indeed this is a very interesting discussion.

As Mike and others suggested, It might have something (or sometimes perhaps everything)  to do with mental aspects.
But I don't think we should underestimate purely physical reactions, which I think Mark has been specifically experiencing during his training.

Unfortunately I don't have sufficient scientific evidence to backup my story completely, but I have always claimed that there are two ways of physically enhancing performance during training of a lift:

1. Increasing neuromuscular efficiency.
2. Lowering the sensitivity of inhibiting feedback mechanisms.

 
No: 1 Increasing neuromuscular efficiency during a session, can be acquired by for example by doing some CAT/speedsets preceding the ME sets.
On the other hand, due to the built-up of fatique, stress and "pressure" (or whatever you want to call it) temporary biochemical changes at muscular/cellular level will take place as well.
Mike refers to this biochemical alterations as "a reflex that enhances neuromuscular efficiency" and I think he's spot on with that one.

Furthermore, If we take a look at factor no. 2:
From my experience (and there is also some research that supports this, I believe) Using a supramixal load (heavy walkouts/partials, weight releasers etc.) or 1-2  heavy singles around 90% 1RM preceding the actual worksets  seems to elicit this kind of dis-inhibition.

There's also a strong possibility that during the succesion of substantial heavy sets a decrease in inhibitive mechanism response is obtained as well.
Likely to be caused by the accumulation of fatique and the induced temporary biochemical changes at cellular level that come with It. 

I don't know how it al interacts exactly, but I do know that making use of these "tricks" in a sensible way means increasing chances of breaking PR's.