To perform this clip you first want to Item Slide into the wall shown in the video. Shortly after starting the slide begin to hold between Up and Up Right on the left control stick. If done correctly, Link will either fall or jump through the wall and you can swim to the Forest Haven loading zone from here.
It is possible to perform a Ledge Clip on the lowest level of Forest Haven and swim under the entrance loading zone to get inside quicker.
A roll clip can be performed at the bottom of Forest Haven in order to clip under the island and swim to the loading zone. However, this trick is somewhat precise and also requires Beedle's ship to move slightly, making it only a small timesaver in a Wind Waker HD speedrun.
Since Wind Waker diving only works in very few places in Wind Waker HD, one alternative to a Wind Waker dive in a Hyoi Pear dive. However, since this requires the purchase of a Bait Bag, this technique is not faster in a speedrun.
In Wind Waker HD, dying will stop any cutscene or loading trigger from being activated. However, due to the rate that health ticks down at (1/4 of a heart per frame) we can damage ourselves right before a trigger begins loading and then die when the game loads the next area or cutscene. In this instance we can skip the Deku Tree cutscene by dying right before the game fades out to load the cutscene while we still have only 1/4 of a heart. Once the fade out begins, death will not stop it. Link will then die in the deku tree cutscene and respawn wherever a savewarp would take place him. By going back inside Forest Haven, you can then acquire the leaf.
Here is a tutorial for a setup that uses adjustments to make the trick have a larger frame window.
There is only one frame for Link to die on, but using different kinds of sword slashes, we can kill ourselves slower or faster depending on the frame we get when we pause buffer. For this setup (that is for the gamepad controller), you will want to slash the last chu with a vertical slash, standing far away from him. Then pull out a bomb, press ZR to place it in front of Link. When the chu stops moving and only oscillates, look for a frame in which the chu is away from Link but only barely moves compared to the previous frame. All 3 frames following that frame will work for this trick.
No matter the frame you get your pause on, you will want to unbuffer and perform a home buffer. If you still see the pause overlay when you home buffer, that means you got the first frame out of the unpause. In that case, perform the following inputs to do the correct slash:
If you don't see the pause overlay when you home buffer, you got the second frame out of the unpause. In that case, adapt and add 1 to the frame you are on to know what slash you have to do. This means you can get a 4th frame if you get a 3rd frame pause and 2nd frame home buffer. In that case, hold ZL, B and to the left of Link to slash the bomb with the correct timing (slash to the left after a left walk).
To remember what kind of slash you have to do, you can try to remember that the control stick inputs needed for the slash go in order clockwise around Link.
Technically, you can still get the trick to work using frame 0 or frame 4, but only if you get the correct home buffer, so it isn't recommended.
There is no slope leading into the water in the Forest Haven quadrant. However, it is still possible to start a superswim there, thanks to Beedle's boat. In order to perform this swim, you will need first to make sure Beedle is in its original position, either by savewarping to get put back on Forest Haven, or by entering and exiting Beedle's shop. Then, make sure to let Beedle's ship move a little bit, as a superswim is impossible from its starting position. Once Beedle has moved a little bit but not much, you can item slide from on top of the right barrel and towards the island to build up speed. Once you have enough speed, you will want to slide into the water on your left to get a superswim.
It is possible to enter the waterfall where Makar is located before the Wind Temple by using only a jumpslash. This is particularly useful if you want to grab forest water first, as you can then follow the water down and use a late jumpslash to bypass the waterfall. Though it is possible to also jump from the platform near the post and get behind the waterfall with a jumpslash from there, it would be faster to simply grapple the post from bellow.
It is possible to item slide from the normal Forest Haven entrance to the island containing the chest with Treasure Chart 3 inside. The boko babas being able to mess with your angle, it is recommended to kill them with the boomerang.
See the Forbidden Woods page.