Many taijiquan exponents mistakenly believe that the softness/yin of taijiquan must be balanced by hardness/yang of an external art.
Their syllabus offers taijiquan alongside external arts such as ju jitsu or kickboxing.
This attitude demonstrates a poor grasp of yin/yang and of taijiquan.
Yin is heavy, dense, sunk, sticky. It is not flighty and weak.
Being struck by a yin punch will feel immensely painful and it will penetrate deep into the body.
That is the yang part.
Taijiquan cannot be 'balanced' by an external art.
Mixing hard and soft martial arts only ever produces one outcome - an external parody of taijiquan.
You cannot experience the awareness, sensitivity, relaxation and flow of taijiquan when your body is repeatedly fighting force with force and being tense.
The true nature of taijiquan is overlooked.