I think the blocking method described actually leads to this : http://en.wikipedia.org/wiki/Retrograde_ejaculation

In which case the semen is still released into the bladder.

It's funny Wikipedia describes this as "When the bladder sphincter does not function properly, retrograde ejaculation may occur." while tantric texts describe it as  degree of subjugation of primal energies and a path to immortality, far from a malfunction., and similar metabolic process is possible for females too.

Well, if the point is to stop semen from leaving the body then it does not actually do that. You just urinate it out later.

But absorption occurs in the colon and in the bladder. Until matter leaves the body it is open to absorption. So it may leave via urine but some of the vitality is absorbed.

But I think if you do that consistently it will lead to weakening of the bladder sphincter. Which could lead to infertility.

Also to take it to the EXTREME wouldn't that mean you could just drink a bunch of semen and gain a bunch of vitality? (Clearly I am being sarcastic don't do that.)

My understanding is that once your body lets go of something it is gone forever. If you re-absorb any semen in your bladder I think that you would just be getting the raw form of it like with food (broken down into basic proteins and sugar molecules.) So even if you absorb any semen in your bladder I think you would gain more just by eating 1 bite of a pear or something.