Mydata
What the instructor did was to clear a bunged-up spark plug. What you do is you set the mag switch to BOTH, rev the engine up to quite a high power setting (say 2000rpm) and lean it back very aggressively. This makes the combustion a lot hotter than would be the case with mixture full-rich and it "should" clear the plug. I've had cases where it didn't and had to take the plug out. Then... which plug? A multicylinder engine monitor makes it easy. Nearly always it is one of the bottom ones.
Leaning on the ground should always be done. The engine cannot be damaged at such low power settings. Exception: taxiing on rough/wet grass; this can require a lot of power especially if trying to initially move the plane after a few days' parking in a soft spot. Then one has to enrichen the mixture.