C-flushing is taking a buffered vitamin C until bowel tolerance is reached (you have diarhhea). It's useful for:
1) Healing your bowel.
2) Flushing bad bacteria out.
3) Increasing your immune system when you have an acute condition.
4) Finding out how healthy your bowel is (the more vitamin C your bowel absorbs, the worse off it is).
4) Finding out how much vitamin C you can have daily (without have diarhhea).