Each UFO comes with a battery where energy can be stored. If you have energy in your battery -- we have two chips on the battery circled below...
...you can take that energy and add it to the base new thrust you got this turn. You can take some or all of it. But you can not put it back. If you take it, you must use it.
So how do you end up with energy in your battery? When you are placing new thrust, instead of adding it to the 3D compass rose, you can add it to your battery. For every 2 thrust put on the battery space in the new thrust area you can put 1 energy in your battery.
Below we are putting two chips on the battery space, along with a couple thrust on the compass rose.
When you move the new thrust up to the current momentum compass rose in order to figure out your movement for this turn, you can move energy to your battery. In this case, the two chips becomes one chip and is added to your battery space.
You can not put only 1 thrust in the battery space in the new thrust area... all energy put into the battery is rounded down and the remainder discarded.
Next, we really will talk about collisions. Or... Tracks. I know completely not the same thing. Maybe I'll tease you forever promising to talk about collisions.