As I understand it, when carpet fibres are wetted they can lose their memory of how they should lie, and as such they can be retrained to lie in another way. In othe wordsyou can then tease out the dents.
The reason for ice cubes is because if you suggest wetting a carpet to a customer you don't want them chucking a bucket of water over the carpet. An ice cube is a defined amount of water and is sufficient to wet the carpet enough to then retrain the fibres.
It's one of those jobs that if customers ask me about I explain how they can do it themselves but if they really want me to do it I shall be happy to do so but as it is a time issue I shall be charging them appropriately.