As I suspected, there is a confusion out there on very basic concepts.
The US Private Pilot PTS/ACS states clearly what is required knowledge and skill for the checkride. The same for the glider pilot certificate. A correct forward slip shall be demonstrated, and if no wind, the sideslip shall be explained. If unable, you may fail the checkride.
In a glider, a skid in the circuit to land could fail the checkride. To demonstrate the difference between skid and slip I have this done in a turn at altitude. A skid + stall (as in base to final turn) will start a spin, a slip, on the other hand, will first level the wings.
This is how I teach the forward slip:
https://youtu.be/yxy2MnUnfUM