As an example, let's assume that the sweet spot is half the lens (in X and Y dimensions). If you render the outer area at 50% res and the inner area at 100%, you've saved rendering half the total pixels, more if you implement a central occluder. The math sounds weird but, that's because of linear measurements vs area measurements.
Let's make the math easy: Assume the screen is 200x200 pixels, which is 40000 pixels. The central area would be 100x100, which is 10000 pixels. If you render the outer area at 1/2 res X & Y, that's another 100x100 "stretched" pixels. For a total of 20000 pixels (or only 20000-(50x50)=17500 with an occluder). If the outer area is already blurry you probably won't notice the lower res very much.
Some implementations draw various areas at 100%, 50%, 25%, 12.5%, and 6.25%, depending on distance from central area.
If done well, this can double your framerate (or more) with little noticeable image degradation. It's simpler than implementing full foveated rendering (since no eyetracking is involved).