how can you have a dry ejaculation while masterbating?

Answer 1

This is not possible. A true ejaculation involes semen escaping and this is in no way dry. The only way that you could do something of the sort is get right up to climax and stop before ejaculation. Which wouldn't be very much fun, now would it?

Answer 2

I believe what you are attempting to describe is something called male continence. this is what it is called when you have an orgasm without ejaculating any semen. the best way to learn how to do this is to look for books or websites that on Kegel exercises. Kegels will teach you how to control you PC muscle (PC stands for a really long scientific name), which can lead you to control ejaculation.
NOTE: it often takes weeks of practicing Kegels to master it.
the benefits of doing these exercises is a much longer sexual experience, many men use it to last for hours with their partner.
here's a quick way to start doing the exercise. next time your urinating, place your fingers right behind your testicles. then stop urinating in mid-stream, the muscle you used to stop urinating is the PC. now every day flex that muscle 20 times at 3 times a day. that will help the muscle get stronger.