in my opinion, this is not a thing you buy, but it's a thing you do.
to "torque stop" anything with an engine, you kill the engines power but leave the clutch engauged (clutch lever released).
this will used the friction of the non running engine, to slow the bike down.
the piston will still be going up & down when you kill the ignition and will slow the bike down.
all you do is hit the kill switch and hold it in with the clutch lever released so the engine doesn't start back up.
the piston will keep moving because the rear wheel is still moving, but the friction from the piston going up & down will act as a brake.
I have no idea if this is ok, or if it's bad for an engine tho.
I personally never do it.