"In midsummer 1946.......When the master taught me, he constantly emphasised the practical application of the art. He himself would act as my opponent and order me to make use of Tai Chi Chuan techniques to disolve his attacks and furthermore to hit back at him. At the time I was in the prime of life being young and vigorous. I found this method to be deeply interesting. Morning and night I stayed with him under a relentless discipline, eager to obtain his secrets."
Cheng Tin-hung 4th May 1961
Cheng has described this experience elsewhere aswell, about sparring with Qi Min-quan, also he himself sparrred with his students up to a certain age then had his senior students spar up-coming fighters under his supervision.
Likewise when I mentioned my desire to fight sanda to my Sifu - Dan Docherty, he directed me to one of his senior students, a fellow Irishman - Paul Mitchel, himself a very accomplished martial artist. At first it was just Paul and me, but gradually others were invited to train. Paul gave these classes / training sessions 3-4 times a week, and on the days he didn't I ran to his house 10km away from where I lived, for 2-3 hours training and ran back. The classes ran for 3 hours and were usually 1/2 hour conditioning, 1/2 hour TCC tuishou and sanshou drills, 1/2 hour technique practice, 1/2 an hour wrestling, 1/2 an hour heavy sparring and 1/2 an hour hand and weapon forms to warm down.
In effect I sparred and wrestled with Paul 6 days a week for 10 years. during this time we travelled to Dan's camps and seminars, met with him at and after competitions, where he saw us go up against fully resistant opponents, and he travelled to Ireland several times a year holdiong weekend or week long camps there too, where he would carefully watch us spar / wrestle etc. and come up with pure gold dust advice and methods that we would apply to our fighting. He would step in and there would be short respectful exchanges. Advice was given once, if he came back a month or two later and it wasn't incorporated (some people just don't practice?) he would never bother giving further advice to such people. Harsh, but traditional I guess?
Now for myself as a coach, I too wrestle and spar my students. When doing so I bring it to their level, it is a delicate balancing act to challenge a student so they improve but not to dishearten them. For this reason I refuse to spar my combat sports athletes coming close to events, immediately after is fine, and can even humble them if they got an easy few wins, but beforehand they need to believe that they are invincible.
Its a tricky job, like Wudang Shan, one reaches a skydoor, believing it to be the summit, only to see stretched out before you another peak to climb. Some students find this disheartening, and feel a lack of progress. I have to remind them that their training partners are also improving steadily, as they do, and this cloaks their own progress. It is only at competiton or when some new lads arrive to train that they realise like Neo - "I know Kung Fu!"