let's say you have 2 takes of a stereo passage up on your screen...
you listen to the first take, discover a rough spot. you want to insert the rough section from the second take, which is a better performance. nothing new so far...
whereas in other DAWs you might copy/paste, with a 4 point edit, you set "in" and "out" points, and then use "insert", or "replace" commands.
so, again.. in the first take, you set your "in" point just prior to the lousy passage. this is your "destination" EDL. then, you find a suitable "out" point, where you would like to return to the 1st take. (typically, you zoom in all crazy, to the sample-level if you like, and find the most suitable crossfade points)
then, in the second take ("source" EDL), you define, with "in" and "out" points, the section you would like to use as a replacement. (you'll zoom in again, and find that exact spot where the takes will line up...)
then, you hit "replace," and in-it-goes. the bad section disappears. the important thing, is that this happens independent of any relative time constraints, or BPM concerns. any small (or large, for that matter) difference in time of the passages is irrelevant, because all 4 edit points are defined. after the edit, the DAW will usually give you a default crossfade that you can then edit to taste, using the crossfade editor (if applicable.)
this is known as a 4-point edit, because you have defined 4 edit points. there is also a 3-point edit, where you don't need to specify the "out" point in the destination EDL. if you don't specify this out point, the "source" selection length will determine the "destination" out-point.
there's also the insert command, which behaves similarly, except... well, it inserts the source material into the destination at the in point, instead of replacing a passage.
the concept is similar to setting an auto-punch-in region (or whatever you call it) when tracking in protools, but you're not recording a new take, you're taking from an existing take.
it sounds very simple, and it is. but it can be a life-saver because of the accuracy. it's a feature that it borrowed from video editing.
i'm really not sure if anything i just wrote makes any sense at all to you. sorry if it doesn't.
FWIW, the "take manager" feature in the latest version of logic looks really neat, and i'm looking forward to checking it out. anyone have experience with that? looks really simple and intuitive.