Do you use it to speed up ur rotations for yourself (To smoke faster)/ your teammates or do you use it when your retaking
Attack: I'll usually use it for the site exec itself. If we're rotating, then it depends... if it's a matter of beating the other team to the open site, then I will, but otherwise I'll save it, especially if it's a quiet rotation in which case there's no point in using it anyway.
Defense: if it's a rush on the other site, I'll use it to rotate because it gets me there that one extra second earlier, which could be enough to change the outcome of the round. If I know that second doesn't matter, then I'll either save it for when the team is going to retake OR just on a whim (whenever I feel combat stim is useful).