What I do is very slightly get out of saddle, less then a cm (pants still touching) or so and unweight and start skid, then GENTLY sit back down. All happens fast, so it "looks" like I'm seated the whole time. I'm thinking thats how the guys in the vids are doing it. Im 175, running clipless and 46/17. Clipless helps, like said before, you can get more power to the pedals. I also don't try to use other muscles then normal skidding, it all about form, not strength