Climb up onto the broken pillar and get ISG. Position yourself so that the right side of your B button is against the slanted line in the video, and then turn around and put on the hoverboots. Do a backflip and pull out a bombchu to start your hover, and then for the next few hovers, pull out the bombchu and once it's above your head, shield drop it and backflip (this will give you more height). Once you are right by the platform, you may have to do one more hover with a bomb to reach the switch. Overall, this trick should take 11-12 explosives.
Note: To get back over the gap, you can do either a megaflip, or use the hover boots and a well timed jumpslash from this platform.
Unlike in the original OoT, this is no redead by the boss key chest in Master Quest to get you past the spiked walls. Instead, get ISG and do a staircase hover. Equip the hover boots, and do 4 bombchu hovers, making sure to pull out the bombchu and then backflip for the additional height. Now unequip the hover boots and sidehop over the wooden wall to the boss key. If you were fast enough, you should have no trouble grabbing the key.
Recently, it was discovered that one can simply weirdshot to the Shadow BK chest. A non-TAS video will be added shortly.