When the treadle wheel turns backward and makes the handwheel go backward, yes the thread will snap or snarl.
When a machine is put into "Reverse", the handwheel doesn't turn backward, the mechanism that drives the feed dogs reverses direction. 2 very different things. Hence a sewing machine equipped with reverse will work just fine on a treadle if it's built or modified (i.e. handwheel changed, etc) to treadle properly.
ETA: This is just like an electric machine. If you throw an electric machine into reverse, the motor doesn't spin backwards. Same with a car. Think of the treadle or the motor as the same as the motor in a car. The feed dogs are the transmission - it determines direction and how fast you're making stitches (i.e shorter stitches are slower with the same amount of power provided than if you're making longer stitches.).
Last edited by ArchaicArcane; 06-07-2015 at 11:44 AM.