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.
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.