Bottle on B is used in RBA (Reverse Bottle Adventure) and is a very well-known Glitch in OOT Speedrunning. RBA allows you to write values to various items such as Deku Nuts, Deku Sticks, Arrows, and so on. But the fun doesn't stop there - RBA can also write values to your B Button, giving Link things like Megaton Hammer on B, Farore's Wind on B, Hookshot on B, etc.
To write things to your B Button - Have Bottle on B with contents and get a specific value of Deku Nuts. Go to the Temple of Time and become Child, equip a Fish on C-Right, then go Adult again; Whatever your Deku Nut value is has been written to your B Button!