Snaffle
A bit with no leverage: the rein pulls directly on the mouthpiece, so the pressure the hand applies is the pressure the horse feels.
The distinction that matters is leverage, not the shape of the mouthpiece. A snaffle has the rein ring in line with the mouthpiece, so a pound in the hand is roughly a pound in the mouth. A curb has shanks below the mouthpiece and a strap or chain under the chin, which turns the rein into a lever and multiplies what the hand does. A jointed mouthpiece does not make a bit a snaffle and a straight one does not make it a curb — the rings and the shanks decide it, which is why a jointed shanked bit is a curb no matter what the shop calls it.