Did it ever occur to you that you could run an ordinary car engine -- cylinders, pistons, crankshaft and all -- with no fuel?
No problem! You just install really heavy-duty spark plugs, and as the piston reaches the top of the compression stroke in each cylinder, fire off a huge spark, think of it as a small bolt of lightning. The air doesn't care where the energy comes from, just that it gets hot and expands to push the piston back down. As long as you dump as much energy in through the spark as you would have gotten from oxidizing fuel, the motor hums right along.
Of course, this would be a horribly inefficient way to use electricity to turn a shaft; existing electric motors are much much better. The point of the mental exercise is to get you to compare the amount of energy used by the spark and that produced by burning the fuel. Realize that they might actually be comparable.
In fact, that's what is going on in pretty much all current fusion experiments. So far, nobody has managed to get as much energy out of the fuel as they put in in the spark. The latest big buzz from the JET torus managed about 70%. However, with the current amount of interest and investment, it seems likely that someone will hit 100% (or exceed! 101%!) sometime this decade.
When you hear about that, what will it mean?
Nice bit of science, not so much for energy. Let's go back to the car engine. 70% or so of the energy produced by the burning fuel is simply thrown away as heat. Then of the mechanical energy you get, you use a lot of it overcoming friction in the engine, turning a generator to produce more sparks, even pumping coolant through the engine so it doesn't melt.
We don't really know in its fullness how the energy from the fusion reactor is going to be turned into (say) electric power, but we can say with fair assurance that it's not going to be cheap, easy, or efficient. So even when the magic 100% figure gets achieved (and for more extensive times than 5 seconds!), it's virtually certain that you have at least another decade to wait before there is any fusion power flowing on the grid.