The MMU, or Manned Mobility Unit, is designed to move you, the user, around in space. This is part SWEP, part SENT, as it is used as a SWEP initially, but has effects like a permanent SENT. When selected from the Weapons menu, the MMU will appear on your back, and you will see the handles.
Primary fire will activate/deactivate the MMU, but only when the MMU is your active weapon. Whilst the MMU is active, whether the MMU is the weapon you are using or not, performing the equivalent of walking forwards will propel you in the direction you are looking at the time. Left and right strafing will propel you left and right respectively, whilst backwards will propel you in the opposite direction to the direction you are facing. Jumping and crouching will propel you upwards and downwards respectively, irrespective of which way you are looking. Pressing the "walk" key (default "Alt") will stop you where you are. Each of these bursts will drain 25 of the MMU fuel, the maximum load being 500.
You can use any weapon as normal with the MMU equipped, the only reasons to use the MMU as your active weapon are to check fuel, activate/deactivate it and to manually refuel. It is recommended to deactivate the MMU on the ground, as it is dumb, and will drain fuel when you move occaisionally
Manual refuelling occurs by pressing reload adjacent to a LS3 (possibly LS2) Energy storage tank. The Energy:Fuel ratio is 100 energy to 1 fuel, so it will be expensive to fully refuel. The MMU will also refuel itself automatically, gaining 1 fuel per 2 seconds or so.
On Death Edit
When you die with the MMU, the MMU will appear to respawn with you, but it does not at current. You will have to respawn it from the Weapons menu.