flex your ankles

first a hard but short stab on descending front pedal then pushdown on rear upcoming pedal while pulling up on the front, or use the rear upcoming pedal to spring off of, works either way

here do this while off the bike:
standing feet together, barely flex your ankles at all but just enough to jump off the ground, do this with your legs stiff, its all ankle power

same deal on the bike

the idea is to get your body weight OFF the bike for a split second, once your body weight is in mid air so to speak you can over power the rear wheel quite easily

once youve done it a few times you can do sort of a rocking motion with the cranks and do a whole bunch of little 6-12" skids and never leave the saddle

it isn't so much a brute force type maneuver but finesse and timing, it really doesn't take a bunch of effort to do it if you have had some practice