particles.setDirection

particles.setDirection sets Direction on a particles object. It takes id, dir1 and dir2.

particles.setDirection(id, dir1, dir2)

function start() {  particles.create('main', 10);  particles.setDirection('main', 1, 1);}

particles.create is there to give the call something to act on; particles.setDirection is the line that matters. The code sits in start(), which the engine calls once, when the entity spawns.