2-person form

Tai chi people usually work at one speed: slow.
Slowness is good for developing muscle tone, balance, control and accuracy; but will fail in combat.

If the art is to be viable in self defence, we must vary the pace constantly; stillness, motion, pauses - with no apparent rhythm.
This should not be a random process, however, for like the dancer, we must be guided by the rhythm of the movement itself and our partner.
2-person form trains broken rhythm in a pseudo-combat scenario.