you scissor your skis on the rail for the direction that you want to spin. this allows your feet to manipulate the way your body wants to spin off the rail. spinning down the hill (frontside) you put your front heel down front toe up, if you spin up the hill (blind or backside) you point your front toes down front heels up.
backside (or blind)
but for a 90 rotation either way off of a rail, you really just need to be balanced and square on the rail and just slightly turn your shoulders. make sure your shoulders are square over your knees as your sliding, then just turn slightly at the end.