This is for the new touch controls for Minecraft PE. If you wear an elytra and press the crouch button with fireworks in the off-hand, it will not work. For example, if you put a shield in your off-hand, pressing crouch will use the shield instead of crouching. Yet if you put fireworks in your off-hand while wearing an elytra and press crouch, it will not use the fireworks instead of crouching. When you are holding fireworks in your off-hand, then crouch while wearing an elytra, it should use the fireworks.

How to do this:

1. Put on an elytra, put a shield in your off-hand, jump off a cliff and fly around, then press the crouch/down-arrow to use the shield. Notice how you can use the shield.
2. Put on an elytra, put fireworks in your off-hand, jump off a cliff and fly around, then press the crouch/down-arrow to use the fireworks. Notice how the fireworks does not do anything.