Q1. Why do Cricket bats need pressing?
To create a hard surface layer off which a cricket ball will rebound. Willow is a soft wood. If it is left unpressed it is like having a set of toilet roll tubes taped together and while the ball will rebound, far more energy is absorbed by the structure than is passed back into the ball. It also protects the willow from damage.
Q2. What does pressing achieve?
Pressing "flattens" the tubular microstructure in the top 5mm or so of the face of the bat creating a hard layer with a higher coefficient of restitution than the raw willow would have. Imagine flattening the toilet roll tubes. The outermost would have the toughest, damage resistant surface and as you move through the depth of the willow, the springiness of the lower tubes would be retained creating a spring like effect.
Q3. What does knocking in do?
Knocking in further compresses the face of the bat and is required where clefts are pressed in an estimated fashion, i.e. in bulk at a similar pressure. It also allows specific higher pressure to be applied to areas where most damage is likely to occur, i.e. the edges and toes, where a uniform pressing across the face doesn't harden the most at risk areas enough.
Q4. What happens to the bat when it is used � why does the bat get better?
Further compression of the willow fibres occurs until an optimum point is reached, where the underlying tubular structure offsets the surface hardness to perfection generating the perfect combination of spring and hardness. After which the willow quite literally falls apart from itself as the bonds between the micro-structure break down and the face lifts off.