Back and forth, no pressure at all. It's basically the same thing as sand paper, but riding on top of a film of lubricant so that only contaminants above the paint surface are sanded away.
Never put any force into it or use circular motions where your application force could vary or where you could dig the edges in. You basically want it to float under it's own weight and stick to the surface only through the lubricant adhesion. Gently and slowly slide it back and forth until you don't feel an grabbing any more and the surface is smooth to the touch like glass. The only force you want is parallel to the surface to push the bar through any snags it encounters (the impurities you are removing), but you want zero down force. Let the abrasive do the work, not your arms. Rule of thumb is just use two fingers, and as if you were moving a computer mouse across a glass table top and don't want to scratch it, and don't let the surface dry out. If it takes effort to push, you don't have enough lubricant. It should glide with ease with no friction, but you'll feel some grit as it's doing it's thing. Work on small sections at a time and remold the clay between sections to make sure you aren't just collecting all the crap and rubbing it into the rest of the car.
So many sexual innuendos here.
Think of it like shaving, you just pull the razor across your face and let the blades do the work, you don't dig in with it.