[elbear]

I believe the benefit to Tai Chi is that you're more in motion. With Yoga you are more static. If you are static in a position where your muscles are contracted, it's hard to relax.

With Tai Chi you are doing movements which contract and relax your muscles for shorter periods of time, which helps you relax. Also, the fluid movements can bring you to a state of effortlessness, which is of course also relaxing.

[cachecrab]

Not all yoga is like that. Yin yoga involves holding one position for longer periods of time (often 2 minutes or more) and is meant to be relaxing.

[kuschkufan]

holding a position sounds pretty static to me

[nonethewiser]

I would agree but maybe he was excluding the forms of yoga that arent dynamic by naming the most static one ...