You place your hands in your pocket, aim the shaft towards the fap rotor, turn the pants/waist so it's closer for your rotor to reach the shaft, then imagine using only every finger except your thumb as if you're playing frets on a guitar but in one big grasp

Like you're telling somebody in hell to "come over here" but your hand is upside down and you're actually cumming in there

Now never move your elbow=dead giveaway to witnesses. Only move your wrist tops, then keep a straight face.

Once the cannon has been fired, just let it dry/cool off. It'll disappear without a stain on your pants assuming you're wearing jeans.

DONT DO THIS WITH BASKETBALL SHORTS THE STAIN WILL SHINE THROUGH LIKE A FRESH MURDER OF A MARSHMALLOW MAN