Further alternative to develop max isometric hamstring strength under more strenuous conditions.

Assume proper form, then explosively rotate your upper body to either side while pressing the ball. The pressing motion creates a further momentum which has to be met with an adequate hamstring response, thus reinforcing robustness of hamstring activity in an unstable environment.

Hold the end-position for the time you need to feel proper hamstring engagement.

T-spine mobility could be a limiting factor performing this exercise.