Block Clipping is a technique that allows Link to pass through blocks in dungeons. This is particularly useful in rooms containing many enemies (such as Darknuts) that must be killed in order to move a block.
This trick uses the same basic mechanic as Screen Scrolling. First, position Link just under a block so that the top of his hat lines up with the "top" of the block (where the color changes).
Once you're in position, you need Link to turn in place in a specific way. There are two methods that you can use. Either:
The second method is a bit easier, but either will work fine. Once Link has turned around, simply walk up into the block. Link should move all the way inside it and be able to walk out the left or right side.