I heard cotto does it and i also read it is better to hit your opponent when he is inhaling air not exhaling, why is this and how do you time it ?

impact is absorbed more when inhaling.

impact is absorbed more when inhaling.

There you go

as you exhale, your thorax pushes out, thus absorbing some of the punch.

Your trainer probably shoulda told you that it's best to take a bodyshot while exhaling.