The pendulum principle dictates that the straps will always want to come back to the axis of symmetry (the midl-line) due to pull of gravity. This means that on SOME exercises the force of the pendulum swing can make the exercise easier, or it can make the exercise harder.
The three types of pendulum effect are:
Changing the pendulum effect on an exercise such as the Hamstring curl can have a significant impact on the total load.
Biomechanics reference for pendulum principles:
Nave, Carl R. (2006). “Simple pendulum”. Hyperphysics. Georgia State Univ.. Retrieved 22-3-2010