di gung training

[Mr. Nemo]

My sifu mentioned this kind of training, which should translate to "ground skill" (right?).

As near as I can understand, it's practicing your style on the ground, particularly power issuing.

For example, practicing on your back, or on one knee. Has anyone gone through this training, or know anything about it?

[count]

That would be Da Tang or Di Tang. It is the groundfighting of traditional Chinese martial arts.
:)

[Kevin Wallbridge]

Actually Di Gong is a valid term as well, it simply means ground-work (Gong is composed of two radicals one for work and one for strength, so the-strength-that-comes-from-work). Di Tang has come to refer more to the falling skills than the grappling work.

"The heart of the study of boxing is to have natural instinct resemble the dragon" Wang Xiangzai