From Sonic Retro
Along with his brute strength, one of Knuckles the Echidna's most obvious abilities is gliding (called the Power Glide by the Sonic Triple Trouble US Manual). Extending his spiked fists in front of himself, Knuckles can slow his rate of descent from height and cover significant horizontal distance. Combined with his ability to climb up and down most walls he might latch on to while gliding, the technique enables Knuckles to explore levels from top to bottom.
The Power Glide seems to be a personal invention of Knuckles rather than an ability of all echidnas from Sonic's world; during Sonic Chronicles: The Dark Brotherhood, Shade is conspicuously lacking in a glide ability where Knuckles can use it, although the Nocturnus echidna does have the ability to climb walls. In Sonic Adventure 2 and Sonic '06, Rouge the Bat glides in a similar way to Knuckles, but Sonic Heroes demonstrates that she can in fact gain altitude by flying properly, as Team Dark's Flight character.