system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is an EMdrive thruster supposed to work?

I saw the article about how it's more successful than they expected, but it doesn't explain how it works.
gpt
Normal rocket engines work by forcing stuff out in one direction very fast. The problem with rockets, is that you have to life the fuel, along with the spacecraft. So if you want to go faster, you need more fuel to burn longer, or you burn fuel faster for shorter. But more fuel means a heavier spacecraft, which needs more fuel.... rinse and repeat.

An EMdrive, in theory, does not need fuel. You're converting electrical energy directly into force, without throwing mass away in the other direction.

So, if your engine does not need fuel, then you can run the engine forever, as long as a nuclear reactor holds out. So even if the engine has low thrust, it doesn't matter. 

The issue with the current EMDRIVE is that the physics behind why it is supposed to work are shaky, and the amount of 'thrust' is still within experimental error.

Supposedly, the engine manipulates quantum fluctuations on one side of the cavity wall. There are virtual particles that pop in and out of existence, and if you can make more of them pop in and out on one side of a wall then the other, there will be a net 'push' on the wall.

But, in a previous experiment, they intentionally 'broke' the engine and it still worked. Which means it's doing something, just not what people think.

So, it could be either the greatest revolution in modern spaceflight and physics we have ever seen, or a bad experiment setup. Time and rigour will tell.