Question

How does gandalf save the dwarfs from the trolls

Answer 1

He brings the light of dawn– in the movie version he cracks a rock in twain to allow dawn through, in the book he is merely a harbringer as it crests the hill.

Answer 2

It is not long before all twelve of the dwarves are held in sacks and the trolls are contemplating another dinner. Gandalf rescues the dwarves with an invisible appearance. He periodically interrupts the trolls' conversation, saying false statements in voices that resemble the trolls' voices. Bert, William and Tom each conclude that the other two are lying and/or mad and of course, they engage in more physical brutality, whacking each other in the head and arguing until dawn is suddenly upon them and they turn into rocks.