The surface of a liquid phase will be in equilibrium with a gas phase. Because energy is exchanged in changing phases, this prevents water from being heated beyond its boiling point. The excess heat is removed by changing phases.
If you heat a liquid and the heat is trapped away from a gas surface, the liquid can be superheated, it will contain more heat than needed to enter the gas phase. If all of the surrounding molecules are also in the liquid phase, there is no release for the energy. If too much heat is trapped in the liquid phase and a gas bubble forms, the conversion to the gas phase can be explosive. That is why bumping occurs.
A boiling stone or stick simply has air entrapped within. It is porous. If you heat a boiling stone, the gas will expand and be given off. If this gas meets superheated liquid, the liquid can equilibrate between the gas and liquid phase, boiling results.
You must be careful with using a boiling stone or stick. If heated too long, all of the entrapped gas will become lost and become ineffective.