Severus feels (and looks) as though Q is speaking a foreign language, but just inclines his head.
"Relax your arm completely," he says, and puts his hand over Q's. "This is what to strive for." He uses Q's hand to slice the cabbage paper-thin, thunkthunkthunkthunkthunk, and then lets go, adjusts a few points of Q's arm and wrist angles and the hold of his fingers, and nods at the second half of cabbage. "Go on."