Half cheec dont apply pressure as the rein and cheek piece both attached to the bit.
Drop snaffles wich the rein and cheek piece attach to different rings to have poll pressure
They can apply mild poll pressure if you use keepers to attach the fulmer cheek to the cheekpiece.
Half cheeks are dressage legal, although whether this is still the case when using keepers I'm not 100% sure - but i think so.