Push....like you have to piss...don't worry, you won't actually piss. Somehow the junk knows the difference. The natural thing is to kinda "suck in", that just "primes your pump". Anyway...it takes some muscle control. First few times it might be kinda awkward, but build up the "feeling" of how to control it and you'll be AMAZED.
I learned this trick listening to some porn guy on the radio....went home, tried it...and goddamn, it works.

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....Now I am the Master