When you working on the arrangement of a song and if you want to make a change to a specific looping region, but then have it continue looping the region before the one you changed. (drum rolls, bass line changes, chord mods, etc).
You can do this with the Pencil tool!
When you have the pencil tool, option click on a looping region and it will turn into a real editable region. What I'll normally do, is turn the loop AFTER the part I want to change into a real copy FIRST, then the region I want to change. I do this so the region will continue to loop as normal after the region I make the change to. For some reason Logic thinks the loop after your edited one shouldn't keep looping, so you have to click it and simply hit "L" for loop so it continues to loop as normal.
Try this trick, you'll be hooked!