It is common for students in a martial arts class to tense-up.
This bad habit is often acquired at a young age.

The problem with tensing is that it locks the joints and stops the skeleton from moving freely.
Instead of falling to the floor in an organic, natural way... the body is stiff, brittle and rigid, and falls like a plank of wood.

There is a Chuang Tzu story about a drunken man falling out of a cart.
His inebriated state meant that he did not realise that he was falling, nor did he tense-up.
Consequently, he was not injured by the fall.
This is not promoting drunkenness or 'spacing-out'; but rather: relaxation.

The drunken man simply went with what was happening - without resistance - and was unharmed.