Impact dents in themselves often happen and there is little that can be done to resolve them. However, if the deck of a board lacks the expected robustness and starts breaking down due to repeated application of pressure, the situation will ultimately lead to delamination. It's only after the delamination has occurred can epoxy be injected with any chance of success. I must emphasize that the fix is only really workable if the area is relatively limited in size, and it's only really a crude stopgap repair. In reality, repairs of this nature often can have mixed results, and it's a usually a strong indicator that one needs to plan on buying a new board.
To inject epoxy into the board, a small hole needs to be drilled into the center of the delaminated area, and a syringe is used to inject it. It's important not to make too hot a batch, as that can lead to other problems.