Welcome to D.va Mystery Egg! In this mode, every player spawns as D.va. When their mech health reaches 0, they will pop out as a random hero with their Ultimate available!
In the top left corner of your screen, there is a Call Mech Charge meter. This will start increasing once you have left your mech. Once the meter reaches 100, you will be able to "Call Mech", and become D.va again, after a small delay. This is interruptable by stuns, so be careful!
When you're still D.va, you can hit the Ultimate button to damage your mech. Do this repeatedly and you will be able to force yourself out of mech at any time.
Underneath your Call Mech meter, there is a meter labeled "Time Until Burnout". Once your Call Mech charge reaches 100, this timer begins to tick down from 8. Once the timer hits 0, you will start to burn until you press your Interact key to get back in mech. Every 6 seconds you spend out of mech, the damage taken by Burnout doubles, starting at 40. (Initially, you will take 40 damage, after 6 seconds you will take 80, after another 6 seconds you will take 160, etc.)
I really hope you enjoy this mode, and if you have any serious issues with it feel free to add me on Bnet and let me know.