I've almost achived this trough tantric muscle controll. The sensation is very intense, almost like an orgasm.
The most important thing is your breathing, then you just contract and relax your genital-muscels. Just like when you're pissing and want to stop the flow. Use the same muscle and switch from maximum contractions, relaxation and semi-hard contractions. The last part is quite hard to learn but it's sure worth it. If it works you can have an almost orgasmig sensation for several minutes. There's alot of information about this on the net. Just search for tantric on google.

Good luck!

I might dig up some old links from my favorites if you'd like.

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