What a boost physically does to a rubber
Section 14.1 described boosting as a practice and 14.2 told its history. The question here is a different one: what happens inside the sponge when it absorbs the product? The answer requires no formulas, but it does require understanding how a sponge is built on the inside.
The sponge of a table tennis rubber is a closed-cell structure — small bubbles of gas trapped in a matrix of rubber. Section 4.2 described how the density and size of those cells determine hardness, elasticity and response to impact. A booster alters that structure from within.
When the product is applied to the inner face of the sponge, it is absorbed by capillary action. The liquid penetrates the rubber walls separating the cells and swells them. The walls become thinner and more flexible, the cells increase in volume and the sponge as a whole expands. That expansion is visible to the naked eye: a freshly treated rubber is slightly larger than before, and mounted on the blade without waiting, the edges overhang the outline. The effect is not only dimensional. As the cell walls thin, the resistance they offer to compression drops. The sponge yields to less impact force, the threshold of the catapult effect — described in 2.4 — falls, and the player reaches the non-linear range of response with strokes that previously did not get there. It is the same phenomenon a softer factory sponge produces, but obtained chemically on a sponge that was originally harder.
There is a second, less obvious effect: tension on the topsheet. As the sponge expands, it pushes the upper sheet outwards, curving it slightly. The topsheet ends up under more tension, like the skin of a drum tuned upwards. That additional tension increases the speed of elastic restitution of the assembly — how quickly the surface returns the ball after impact — and changes the feel of the contact: the rubber feels livelier, more reactive, with a more pronounced click on the stroke. On rubbers with a tacky topsheet such as the Hurricane, that tensioning partly compensates for the original stiffness of the rubber and widens the response window 14.1 described as necessary for the rubber to work as designed.
Both effects — a softer sponge and a tauter topsheet — act in the same direction: more ball speed, more sensitivity to the spin generated, a more progressive response curve. A treated rubber behaves as though it were several degrees softer than its spec sheet indicates. That is why combinations presupposing a boost — such as the Chinese profile described in 12.4 — start from a high nominal hardness: the treatment brings it down into the intended working range.
What a boost does not do is last. As the product evaporates or migrates towards the surface, the sponge contracts gradually. The cell walls recover part of their thickness, the hardness rises and the tension of the topsheet slackens. The process takes weeks or months depending on the product, the ambient temperature and the thickness of the sponge. The bat that on the first day responded with elasticity and easy catapult hardens until it approaches its original state, as already noted in 11.6. That progressive decay is the physical trait that most conditions the practice of boosting: it is not a stable modification but a transient state requiring periodic reapplication.
What happens with the specific substances — composition, brands, methods of application — is dealt with in 14.6. The effects of that cycle of swelling and contraction on the integrity of the rubber, in 14.7. What matters here is the mechanism: a boost adds nothing to a rubber, it redistributes what is already there. It swells the cells, thins the walls, tensions the surface. The result is a sponge working below its nominal hardness and a topsheet that restores faster. The physics is simple; the implications are not.