5 bows

Bow tension must exist in your body at all times when doing tai chi. Without it, your power is limited. Bow tension is a refinement of inherent peng.
Stretching assists with bow tension but this must be coupled with spiralling for the power to build.
Skilled use of kwa will enable you to maintain the 5 bows yet move the limbs in a flexible, coordinated manner.
If the kwa are allowed to open and close of their own accord, they imbue every movement with additional power.

Kwa provides the necessary flexibility to reduce the possibility of jarring.
If your joints flex naturally, you find it easier to adapt to physical difficulty and find opportunity for movement.
The kwa help you to create space.