there are different 'movement types': One is rotating the hip axis against the shoulder axis, the other is not doing that.
In fact, responsible for the hardness of a spike are:
- Arm extension out of the elbow joint - 46%
- Rotation of the arm out of the shoulder joint - 23%
- Rotating thorax/shoulder (picture right) OR shifting thorax/shoulder (picture left)- 15%
- Movement of body's balancepoint (in space) - 8%
To get the optimal spiking hardness out of your player you have to first recognize his movement type and then work WITH it, not against.
You cannot change those movement preferences.