The claim is that for some energy input in a certain time, you get a proportional force. This is a linear relationship.
because Newton's 2nd law: F=ma, the relationship to velocity is also linear. Add one Joule of energy, go 1m/s faster. Or 0.000001m/s faster, it doesn't matter.
Now, kinetic energy is given by
E=½mv²
The important bit is the v². The relationship is not linear but quadratic. Something moving twice as fast has four times as much kinetic energy.
So here's how to make an infinite energy machine with one of these drives:
Take a regular electric generator.
Mount a cross-bar on the axle.
Put the magic drive on one end of the bar.
Turn it on. The axle starts turning.
When the kinetic energy of the spinning axle is greater than the energy input of the drive, connect the two together. Now it will get faster forever and you can attach a second generator and extract as much energy as you like so long as you don't slow it down below the breakeven point.