How to use an item in my off-hand?

As of snapshot 15w31a, you can hold things in both of your hands.

I figured out that F lets you hold the item in your other hand. But how do I use this item? Can it even be used?

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It is distinguished by an outline of a shield like symbol.

Pressing F (by default) will switch whatever item is selected in the hotbar with the item in the off-hand (including no item).

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