When:
Anytime from the start of the game, after reaching LEVEL 12 in a run.
Method:
Destroying light sources has a small chance to spawn a temporary power-up instead of the usual coin / chicken.
One of these power-ups is the Vacuum, an item which sucks up all experience gems left on there ground and gives them to you. Collect this! The first time you do you'll unlock the MAGNET for use in future runs.