you've got a couple ways to do this.
You can use your thumb and wrist which is how most of the greats do it. It just takes a lot longer to get down.
or you can flip your hand over and use your pointer and middle fingers alternating to push the stick down.
I generally use moller though.
And lastly you can use one handed rolls.
stick is at 90. arm at zero.
throw the bead of the stick down for the first hit.
your hand should be at a downward angle and u should let the stick come back up to 90.
the key is to not choke the stick and stay relaxed.
the second stroke happens during a combination of your fingers and your wrist coming back to the starting point.
Utilize your thumb to get a little more power and control in the strokes.
I was refering to the buddy rich style roll. Which i can't do YET so heres a vid with explanation.http://www.youtube.com/watch?v=A-4s2yN6PIE