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.
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.
If you go through the exit the game normally expects you to go through after getting the Leaf, you can [Item Slide] into the entrance of Forbidden Woods by backing up against the left wall, charging an [Item Slide] against it and then slash forwards. Unlike what the video says, holding Up is not required when buffering the slash.