yoko guruma - http://judoinfo.com/images/animations/blue/yokoguruma.htm
I actually use this in a slightly different manner, and I do it no-gi, although it will work just as well with a gi.
Get an overhoook on your opponent. So, if you have the overhook on your left side, step back and to the left with your right foot (like a kata turn), do this forcefully, which should put you nearly parallel to your opponent. Immediately shoot your right hand in-between your opponents legs. This should be done in a rolling motion - once the hand shoots between your opponents legs (maintain the overhook) you fully commit to that movement and it becomes a sacrifice throw. The combination of maintaining the overhook and the rolling motion will take your opponent over, and when you complete the roll, you should be in side mount. The circular path your body travels is almost identical to the motion illustrated in the link above.
I can actually hit this move quite a bit, one of my favorites.