If you use a firework rocket, then land and re-deploy elytra quickly, what is left of the boost will still work and propel you forwards. Easy to reproduce if you use duration 3 fireworks, then jump and active elytra mid jump, then use a firework and then land and activate elytra (by jumping and activating mid jump) again before the timer for the firework rocket is up. I believe this to be a bug but potentially not.