I may have read about this feature, and I think I'm experiencing it in example files that come with the Meyers' book, "Creating Motion Graphics with After Effects, CS5."
I am scrubbing along the timeline and there is a tiny pause -- or hesitation or detent or whatever -- at certain points. I think I want this to always happen!
Of course it could be bumps in my mousepad or my imagination....
I found it! I realized the keyword was "snap."
"Shift-drag the current-time indicator to snap to keyframes, markers, In and Out points, the beginning or end of the composition, or the beginning or end of the work area."