A straight slot may perform on a single pull, but the angled slot serves to allow different mechanical effort along with the spring over the weights. This can provide extra force to return to initial travel when you want to accelerate, climb a hill etc. Also a lot depends on the front pulley also. It's ramp angle and size can cause undesirable "non linear" travel as the weights move and the belt rides to the outside of the pulley. This can create a sort of runaway action which will surpass the optimal gear ratio for the speed of the bike and engine rpm. Angling the torque driver can fine tune that action.