A new English willow bat is not finished when you buy it. The willow has been pressed at the factory, but the surface fibres are still brittle, and the first genuine yorker on an unprepared toe can split a blade you have owned for an afternoon. Knocking in is the quiet apprenticeship every good bat serves.
Start with a light coat of raw linseed oil on the face, edges and toe — a teaspoon, no more, rubbed in and left overnight. Oil feeds the fibres so they compress rather than crack. Two coats over two days is plenty; a soaked bat is a dead bat.
Then the mallet. Begin gently across the whole face, working the edges last and hardest, because the edges are where willow is weakest and where the modern bat carries most of its wood. You are not hitting the bat, you are pressing it — rounding the edges, driving the surface fibres into a springy skin. Give it hours, not minutes, spread across a week. The sound changes as you go, from a dull knock to a firmer, tighter note. That note is the bat telling you it is ready.
Finish with throw-downs and old balls in the nets before you ever face a new one in the middle. Knocking in is dull, and that is precisely why so many good bats die young. The players whose willow lasts a decade are not luckier. They just did the boring part.