Selecting the starting point for playback
Say you need to hear a specific measure once or twice to be sure it convinces you.
You select said measure, hit space (or whatever custom shortcut) to start playback, and space again to stop after you've heard the part you were interested in. If you are still undecided and need to hear it again, you click on the already selected measure again and hit space again to reproduce it one more time from the beginning.
The thing is, this has some lag, and depending on the speed at which you do this MS responds differently:
If the time between stopping playback for the first time and clicking the already selected measure for a second time (so as to select the starting point for future playback) is too short, when you later on hit space for a second time it will resume playback from where it had stopped, instead of playing this measure again from the beginning.
If you give it just a slightly longer fraction of a second between stopping playback for the first time and clicking the already selected measure for the second time (so as to select the starting point of future playback), when you hit space for a second time playback will start from the beginning of this measure and not from where it had stopped (which is what one would want).
So you have to get used to giving it a little while between stopping playback and selecting the starting point for future playback. But you are never sure how long, and end up needing a few tries for every repetition.
It makes no difference when it was that you stopped playback the first time: either before the end of the selected measure or after it. All that matters is the length of time between stopping playback and selecting the starting point for future playback.
It also makes no difference if instead of selecting the whole measure to be played from the beginning you select the first note you want to hear or the first barline of the passage or whatever.