In a 2008 Stanford University experiment tai chi master Chen Xiang generated a force 14 times his body weight when striking.
0-60 mph in less than 3 seconds.
An internal strike is unique because it emulates the action of a whip.
There is no pushing.
Any appearance of hardness is caused by the inertia of the whip being pulled back upon impact.
The whip itself is soft at all times.
There is no need to lock the joints or tense the muscles.