Thanks again for your comments, you've given me some things to think about and reflect on in my own practice. From my naive understanding, the balance of li and jin in energy work seems to have a long history in taijiquan and other styles of kungfu, and the use of stone locks, balls and other weight devices have been reviewed by others over the years. My understanding of using a 'material' ball is with an aim of eventually not holding a 'ball', while holding a 'ball'. Perhaps though, I'm digressing from the original intention of this thread.