Here's how to do it: Ride at a slow to moderate speed, with your feet in the ollie position but with your back foot right on the tip of the tail. Snap down on your tail, and start your backside 180 ollie, but after you reach about halfway, kick your backfoot behind you to shove the board, causing the 360 pop shove it. Timing is very important when your doing a bigspin. After the board has spinned a 360, and your body has turned a backside 180, catch your board and land on the trucks.
After reading this trick tip you'll probably have no idea what the bigspin looks like, because it's very complicated. Watch people perform this trick and you'll be doing them in no time. Many people find that performing this trick fakie (backwards) is somewhat easier.