Spokes of a Wheel
Imagine your pelvic and abdominal muscles as the centre of your body - connected to your legs and arms by the muscles and tissues of your body radiating outward. So your lower abdomen (the Tantien) is rather like the hub of a bicycle wheel - connected by spokes to the rest of the wheel/body.
If the spokes are too tight (too much tension) then the wheel has little give in it – little flexibility, the ride is very harsh and movement is tight and jerky.
If the spokes are too loose then the body is simply not connected and movement/power cannot move from one plane to another, e.g. from one spoke to another or from legs to arms. So we talk in terms of 50% relaxed – a state of supported preparedness where the pelvic/lower abdomen is at the centre and movement one side of the wheel (body) can be transmitted to the other side and controlled by the hub (the pelvic/lower abdomen – hips/waist/tantien).
Rotation of the hub (tantien) enables power fed in at one point to be deflected and re-directed at an angle to a spoke or spokes of choice on the other side of the wheel.
This ability to re-direct the incoming force at will by adjusting the position of the hub and tensions of spokes means that our training partner or assailant never gets the result they expect and so become constantly off balance.
Equality we are able to adjust our posture and balance to accommodate other external forces. Thus in everyday life our bodies can also behave more flexibly and fluidly, retaining good posture and reducing our vulnerability to trips and falls, our bodies can remain softer and more resilient – which is one aspect of the health/longevity benefits attributed to Tai Chi.