Rewinds time by a few seconds
Justin first uses the spell in "First Kiss" after their crystal ball is accidentally broken. Jerry instructs him on how to execute the spell, and mentions that it's often used to solve a mistake in the past. He tells all three kids to never use the spell, and not ask any questions.
Alex later uses the spell multiple times in order to help Justin get his perfect first kiss with his girlfriend. With each times she casts the spell, back at the Waverly Sub Station, she unknowingly ruins her family's chance of becoming the official sandwich of the New York Mets.