hunh, ive never actually heard of people doing Ms like that. i dont see why pulling with the index finger would be bad.
but if you dont like it, an alternative to pulling the M slice down with your index finger is using your ring finger to flick DF to DB. i would use my left ring finger to do this because i only do U2 double flick with my right hand, and i do M2 with my left ring finger followed by my right pinky. the key to good finger tricking is finding tricks that will always work well together (something which i, unfortunately, have yet to master).