This is unlocked near the beginning of Level 2, "The Sea".
Once you've made your way into the treetops, you can leap and catch hold of a kite. The secret is partially obscured by some leaves, on a branch near the top of the screen, but in this video you can hopefully see where it is.
If you pass the collectible, crash the kite (either fly into some branches or go off the bottom of the screen) or hold down on the d-pad to reset to the checkpoint just before you jump onto the kite. (Thanks Phase II for the reminder.)
Apologies for the admittedly poor narration, I stripped this video out a Twitch broadcast but it should still serve as an adequate guide.