I've never learned slappies, for me it feels like riding against something and hoping for the best, possible slipping out and just dying. I'd rather skate a knee high ledge or rail because I know exactly how my trucks are going to land on it and grind.
Using so much force by smashing into it, while still controlling the movement is something I don't understand. The closest thing I can do is a wallie.
I wish I could slappy feeble, must feel awesome