Hi Manu!
Good explanation above by Bownarra.
When a stick is bent, there is compression on the belly side and tension on the back side. Somewhere in between is the neutral zone. This is not in the exact middle (depends also on wood species), because wood can better absorb compressive stress and can only be stretched to a very limited extent. This means that almost all the energy is charged via the belly side cells by pressure. If there is a weak point somewhere, the stick will bend too much there, as Bownarra has already explained. If no countermeasures are taken, there will be too much stress and set will develop. Set is caused by overstressing the wood cells, they lose their spring force and do not regain their original shape when stressed. This is accompanied by the fact that the energy charge can no longer take place as it should, at least the first few inches during draw out store significantly less.