The point of no return is the moment a guy feels his orgasm coming. It is inevitable: it will come whatever he does. This is a few seconds before the actual release.
What the guy feels is the contraction of the prostate gland and the seminal vessels. Once this happens ejaculation is on its way and the guy can do nothing to stop it.
Before you reach the point of no return you can control the level of arousal. When you become very excited but don’t want to come yet, you can make behavioural adjustments in order to postpone orgasm. This lengthens the plateau-phase, as you can see in the fgure on the right.
Men who come too quickly often don’t feel this point of no return: orgasm sneaks up on them. They don’t know how to stretch the plateau-phase, as can be seen in the figure on the left.

See also: premature ejaculation.