I probably first heard of Theory of Constraints in Mark Graban's podcast. One of his guests spoke with veneration of Dr. Goldratt's visit to their firm, where he delivered one of his last presentations short before passing away in 2011. I watched the presentation, and also watched the film adaptation of his popular business novel 'The Goal', which gives you an overview of what TOC can do for you.
To give a brief overview, TOC sees reality as processes just like Lean, but its efforts are focused on finding the 'bottleneck' or 'funnel mouth' that every process has, as that is the 'weak spot' that will condition the throughput of the whole system.
The funnel image comes here very handy: if you try to increase the capacity of a funnel only by pouring more water into it, all you're going to do is overflowing the funnel. Everything is useless unless you widen the mouth of the funnel.
And that is true of any process; you have to investigate where's that funnel mouth. And once you find it, all your improvement efforts should be focused on that point, as any improvement in its capacity will benefit enormously the whole system. Contrarily, efforts that are not focused on that spot will hardly have any effect on the bottom line.
Once you've located the bottleneck, TOC also provides a recursive 5 steps process you can use to exploit to the maximum that constraint, and finally transcend it by moving it elsewhere.
This was, one day came to me, the way out of my "random access Lean" (I have to say that the insight of how both techniques interact is not an act of invention, but a recognition: it is Dr. Goldratt himself, and other TOC specialists, who affirm that Lean tells you what to do and TOC where.)
My encounters with TOC have been more random and spaced than with the other techniques and, given that in this field there is zero documentation regarding personal productivity, I'm winging my way more than even. My processes are still not even stabilized (in other words, I still haven't flowed a complete song from one extreme of the pipe to the other), so my determination of the constraint has been so far plain basic guesstimation. Perhaps, by now, my TOC implementation ranks only as 'poetic', and only gives me what Deming would have called 'appreciation of a system'; a reminder that says "as you do this stuff, remember that you are within a system, which is made of parts, and one of those parts is a bottleneck".
Doesn't seem like much, but once you realize you are within a system, you can start to act on it and improve it; and that's much more than what my 20 years old me could say. I think he would have enjoyed this stuff :).