The Candy Crush Saga free switch Hand Booster can be bought and used at any time during the game.
You buy them with FB credits.
It allows you to switch 2 candies that don't match.
It is very useful for getting 2 special candies together to make a powerful combo!
Here is a demo video which shows how it works.
I used it in the last 2 moves in this video to complete the level..
The video was made on level 28.