Nine in the third place means:

Keeping his hips still.
Making his sacrum stiff.
Dangerous. The heart suffocates.

This refers to enforced quiet. The restless heart is to be subdued by forcible means. But fire when it is smothered changes into acrid smoke that suffocates as it spreads.
Therefore, in exercises in meditation and concentration, one ought not to try to force results. Rather, calmness must develop naturally out of a state of inner composure. If one tries to induce calmness by means of artificial rigidity, meditation will lead to very unwholesome results.