So, what is a block clip? Well, if the player manages to be on yoshi, and be big, there is a "gap" between their hitboxes. The gap happens to be ~2 pixels large, and although that number may seem small, that means you have 32 possible different ways of clipping a block from its bottom.
To easily perform a block clip, you can enter donut secret 1, where all that is required is swimming into the bottom right hand corner of a block, aligning yoshi's neck with the corner. It can also be done from the left side, but it is a lot more precise.
The trick on land is more precise, but still quite easy to perform from the right.
From the left is where the "upwards corner clip" gets its name. When you are at max speed, you can block clip a block from its left side. The only way to do so is to gain speed in the air while you are clipping the block at max speed. It does not require a specific speed, just one that is large enough to shove you through the block.
SMV Used For This Example