Knee roll -- The thickened part of the pad that protects the batsman's knee. Most usually spoken of when commentators use it as a crude guide for judging the height of a possible lbw: a ball that hits the batsman above the knee is likely to pass over the wicket.

Knocking in -- Conditioning the surface of a new bat by repeatedly hitting it with an old ball or a wooden mallet. A bat that hasn't been adequately knocked in will splinter under the impact of a new ball.