https://wiki.teamfortress.com/wiki/Unused_content#Upgrades
>Gives the Pyro a rocket pack that sends it flying in the direction you are looking. When you land with this active, fall damage is nullified, and Robots in a 192 Hammer unit radius will be stunned for 1 to 3 seconds, scaling based on how much fall damage would've been taken; stomping a Robot increases the stun time to 5 seconds.
Pretty cool that that they reused a scrapped MvM upgrade as the basis for the Thermal Thruster (probably)