It basically is when you are approaching a jump (ranging anywhere from just the lip of a rail, to some 80 footer) and you make a snowplow or a slight hockey stop to slow down your speed. Judging the correct speed can sometimes be pretty difficult. You have to take into consideration the wind, temperature (with changes the snow speed, and your distance and angle of the inrun. Knowing the correct speed can be judged by either watching someone, experience, or just sacking up and hitting it, if its your first time ever hitting a jump of that size. Just make sure to be careful, and not be a straight up dumb shit about it.