If you are sung, you are loose, if you are loose like you are supposed to be loose you can strike with effortless speed and explosivity. If you train in your forms and various jiben gong you will be flexible. Taijiquan is a complete system. Those who train correctly should have no problems with flexibility, I don't.
The various positions in the form, such as sitting like a mandarin duck and snake creeps down, naturally encourage flexibility.