The 13th principle of the 13 postures is called 'central equilibrium'.
One aspect of central equilibrium is the notion of core stability.
Stabilising the centre does not involve any form of tension.
It is about awareness, rather than doing.

By allowing the pelvis to relax naturally and being conscious of the body in motion; a tai chi student can develop 'core stability'.
Core stability means that the pelvis feels to stay where it is throughout your practice, rather than tilt or twist.