If I have a shield in my off hand and I crouch to go into the sheilding position then I hit anywhere (e.g on the ground or at a player) then it goes into this mode where it keeps bobbing up and down rapidly on your off hand. While it is in this state, you go into your inventory and shift-click on the shield to put it into your inventory slots it will move the item then reappear into the off hand and keep the one in the inventory slot. You can then give the shields out or put them in a chest, or keep them for yourself. In the video I linked, It showed 1 shield starting off with and duplicating into multiple.