This trick lets you take the Fishing Rod outside of the fishing pond area, essentially "stealing" the rod. There are several methods that can be used to perform this trick, but only one method works in all versions of the game.
On it's own, the Fishing Rod is useless, since casting it outside will cause the game to soft lock as the fishing line won't load. However, if you pause the game you can set the B button to have the item stored at a particular address that either stores your previous B item, or whether the B button can be used. By dimming/undimming the B icon before pausing, you can erase your previous B item, allowing you to convert the Fishing Rod into two other items. You can also convert the Fishing Rod into a B Bottle.
See Bottle on B - Shield Swipe Method for more info.
After stealing the rod, if you dim the B button icon and pause the game while the icon is dimmed, the fishing rod will become a blank B button, making Link swordless.
After stealing the rod, you must first dim the B button and undim it, then pause the game to convert the fishing rod into a Deku Stick. Otherwise as soon as you pause with the fishing rod on B, it will revert back to the B item you had before you started fishing.
Once you have a Deku Stick on your B button, you can then use the Bottle B trick to get an empty bottle on the B button for use in Bottle Adventure or Reverse Bottle Adventure.
There are several ways you can dim and then undim your B button. The most common way is to enter a body of water to dim, and exit it to undim. You can also use ladders as well, as climbing will dim the B button, and getting off will undim it.