The actual moment of impulse with the roller on the impulse pallet.
The impulse force that is given is not uniform, in so much that equal force is not given to the pendulum through-out the time the roller is running down the slope.
Ideally the middle of the impulse force should coincide with the mid point of the swing of the pendulum, which the Synchronome is designed to do. If it does not, you get what is called escapement error.