well in a nutshell, all the THC you inhale is absorbed into your lungs within seconds of you inhaling, so the longer you hold your hit, the more carcinogens you absorb into your lungs.

so really all you need is to take a normal breath, almost like smoking a cig.
hold it any longer and all your getting is resin and all that good stuff.
helps to keep the lungs as healthy as possible w/o a vap even though smoke might be a problem for some.