From Sonic Retro
The Hammer Jump is one of Amy Rose's techniques, useable in Sonic Adventure, Sonic Advance, and Sonic Advance 3. The attack sees Amy smack her Piko Piko Hammer into the floor in order to launch herself high up into the air.
In Sonic Adventure, the Hammer Jump required that Amy be running at full tilt, though in the Advance games this requirement is dropped and the move can be used even when stationary by pressing . In Advance 3 it is only available when Amy is teamed with Knuckles. The move catapults Amy up much higher than any of the characters would normally be able to reach by spin jumping, and especially in Sonic Adventure is invaluable for launching Amy up and over obstacles.