Ordon Gate Clip is a simple trick that grants very early access to Faron Woods and Hyrule Field. Several sequence breaks have been discovered as a result of this trick.
There are several rocks strewn along the path to Eldin Springs/Faron Province. Pick up the first one on the right and carry it down the path.
Place the rock in front of the left or right gate door, next to the center posts.
Stand in front of the opposite gate door. Make about a 45 degree angle with the door and the rock and sidehop between the rock and the gate. Jumping over the rock should push Link up and into the small opening in the gate.
Roll towards the center of the gate. To make Link bonk out onto the desired side of the gate, use the minimap to orient the rolls opposite that direction. Rolling at about a 45 degree angle with the gate, Link should eventually bonk off of the center and be pushed back out to the opposite side of the gate. Sometimes he ends up back on the original side; if that happens, just try again.
Get on Epona and ride out to the gate.
Turn Epona around and back up as far into the middle of one of the gate doors as possible.
Quickly pull back on the joystick to make Epona stand on her hind legs, then press R and A to backflip off of Epona into the gate.
Roll towards the center of the gate. To make Link bonk out onto the desired side of the gate, use the minimap to orient the rolls opposite that direction. Rolling at about a 45 degree angle with the gate, Link should eventually bonk off of the center and be pushed back out to the opposite side of the gate. Sometimes he ends up back on the original side; if that happens, just try again.