This technique uses Quick-Equip Canceling to allow Link to clip into walls while swimming. The clipping is achieved by having Link get stuck between an ice block submerged in water and a wall/corner.
To perform this trick, you must be near a body of water with a wall decently close to a corner. Then use the Ice Rod and line up the ice block as close to the corner as possible so that it can float in the water. You then need to QE-cancel your Ice Rod use and as soon as possible, walk off the ledge and swim underneath the ice block so that it lands on top of Link. It might help to mash the B button to be able to make it underneath the ice block in time. Depending on the scenario, you may also need to face a certain direction to achieve the desired clip.
The main reason this trick works is due to the ice blocks gaining a hitbox while they float in water. This can be easily seen by dropping one into water and swimming into it. This is very similar to Tornado Rod Lifting in the sense that Link and something else's hitbox overlap causing Link to be pushed out of bounds.
This trick can also result in Link walking underwater depending on the way he clipped out of bounds (video TBD).