Got this from Stan, who flew 38 in ETO.
A cloverleaf was simple maneuver to improve the turning radius of a fighter. Pull the nose up to bleed of speed which allows a tighter turning radius. Drip the nose to pick up speed, and hopefully, the tighter turning radius has improved your position behind your opponent. Continue this maneuver, as needed, until you've shot the bastard from the sky. If you were able to trail smoke, the maneuver would appear as a cloverleaf in the sky. This was a common tactic used by P-38 drivers, and it was quite effective.
stan r