Location
Found in the Orange Grass area after defeating Jin, after some platforming at the end of the path down and to the right just after the boss.
Properties
As stated on the tin, allows Shin to double jump.
Double jump recharges on touching the ground, touching a springboard, or being pushed upwards in the air by a Pinwheel.
Trivia
It is possible to get additional "free" double jumps by pressing jump at the right frame while rising up through a platform. Can be especially useful during the Child fight.