I always like the way Zhu TianCai does his forms. Zhu Tiancai is one of the Big 4 of Chen Taijiquan from the Chen village. The other 3 are Chen XiaoWang, Chen Zhenglei and Wang Xian.
In this short clip filmed at an informal setting, we see how he can do the New Frame, XinJia in a smooth and yet aggressive way.
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Taiji balls have been around for a while although in the past heavy concrete or marble balls are used as training aid to develop strength. Recently I saw plastic gym balls being used. here is one, you can push hand with a ball. Buy a 56cm diameter ball http://www.stonetempletao.it/pagine/video_streaming_pace_palla.htm
Here is another one on a well designed Japanese site  (as read from Formasa Neijia site)

Have you incorporated other kinds of balls into your training? Let me  know if you have.