Restricted Items (RI) is a trick which lets you use items in areas that you normally can't, such as Farore's Wind outside Dungeons, Ocarina in boss rooms, etc. This trick is done by equipping a item which usually gets dimmed over an item which is undimmed while in certain states. There are a few methods known to do this.
With a Blank B button, climb on a ladder or other climbable wall (note: it must be one that you can climb down to the floor, not fall off at the bottom), then equip the restricted item over an item which is undimmed. When you climb down the ladder, you will have 1 frame to use the restricted item. If you attempt to use Farore's Wind the frame after climbing up a ladder, the game will softlock.
Simply get Down A when you have a blank B button and you will be able to use restricted items by equipping them over undimmed buttons. This method doesn't work if you have something other than 0 or 255 on Temp B.
Get on Epona with a dimmed blank B button and you will be able to use restricted items by equipping them over undimmed buttons.
Get on Epona with a dimmed B button by doing RCE and going under water with iron boots, then pressing A to get back on epona. You will be able to use restricted items by equipping them over buttons which were undimmed before getting on epona.
Climb on a wall or ladder with a blank B button, and equip a restricted item over an undimmed item. Have a text box appear while you are on the wall or ladder, then climb off the ladder and you will be able to use the restricted item until the text box disappears. This method is most commonly done in volvagia's room with the heat text by unequipping the Goron Tunic, which allows you to use ocarina or Farore's Wind on the blue warp. This can also be done with other text boxes, such as a cucco hatching or a gossip stone.
After a wrong warp, you will be able to use restricted items by equipping them over undimmed buttons. This does not work after Blue Warp Cutscene skips however, such as in the Forest, Water, Shadow, and Spirit Temples.