Technique: Corner Boost while swimming downward

    As we know, Mario can get boosted by a solid block if he's swimming (without holding an item) and goes upward fast. This described here and if you don't know what it is, you should read it. The news is, if proper subpixel manipulation, one can do the same trick downward.

    The reason why it's much more difficulty to do it while descending is due to the nature of the Mario-block interaction and because it's easier to go up when you want. If you are below a block and near the corner, there's an interval of 5 pixels in which you can jump and get boosted. However, above the block, the interval is only 2 pixels wide. The vertical distance from the top or the bottom of a block and the pushable zone is 6 pixels.

    Based on this, let's use a block in which Mario can stand on it until x = 314.F and y = 320. Using the 1/1 swimming technique, one would achieve:

    However, by abandoning this technique during a cycle, the positions would be:

    In the 4th screenshot of the 2nd sequence, Mario would go to x = 316.F. But, as he has descended enough (more than 6 pixels after y = 320), then he is in the pushable interval. If it were 317.0, he would be out. That explains why he went to 317.F immediately, obtaining a pixel of advantage.

Prerequisites:

1. Horizontal position: must include the first or second subpixel in which Mario falls. In the example, 315.0 or 315.1, depending on the horizontal speed.
2. Vertical speed: must be higher than 32.
3. Vertical position: better include passing a few subpixels higher than the floor. In the example, the best position is 319.F.

Observation: in the rare cases where the vertical speed is higher than 48, it's easy to do it without any subpixel control.

Edit: this technique messes up the synchrony of the 1/1 swimming together with corner boosts upward.
Donut Secret 1 movie / comparison ghost (amaurea script).
Therefore, it's useless in Donut Secret 1 and levels alike because:

• you don't do it twice without any corner boost upward between.
• when you get out of a pipe and start 1/1 as soon as possible, you luckly get only even x positions ending with subpixels 0 and F. Getting boosted only once changes that.

Anyway, it's good to try it if you don't start the level in a pipe or if some event make you stop the 1/1.