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I cant say there is a definate technique there but if your gettin hirt from spinning them then they might not be the right type of cymbals.
Are the atchual marching cymbals?
My cymbal flashes are wanna be cymbal flashes. We did it mostly for show and yet they werent correctly done, that and we also had a pair of concert cymbals that did that and since the not tied in them wasnt correct they offten got dropped..
Anyway I cant really help on the technique, i'd say for sure thought make sure there atchual marching cymbals.

I've played for four years now. Spinning isn't hard but it does hurt. like burning for days hurts. You need to have ball straps, (ye I know it sounds dirty...) But they really help in not screwing up your straps. Other than that, it's like spinning a rifle that can't come off your hand. The same technique, but you understand...

Your cymbals might be too big for your arm. What size cymbals and how tall are you?

If your taller than 5 foot and have any smaller than 16 inches, it might be the wrong kind of cymbals. (As most have said...)

You might also want to polish technice as well. You're putting your arm straigh or what? Practice does make perfect, but not if you're dong it wrong.

I'm sorry if it seems harsh...