If you cannot gauge how much power to use, you are a liability to your practice partners and need to slow down until your sensitivity develops.
The use of muscular strength is the main sign of incompetence.
Tensing your arms, and shoulders means that you are struggling with the attacker, and this is not the way.
Only by letting-go can you hope to make progress.
Tensing blocks movement - both your own and the attackers.
What is the benefit of this?
What is the point of this?
You must be fluid and flowing like water, not stiff and unyielding like a robot.