The “forced ferment” test tells you what the maximum attenuation you could get for that wort and yeast combination is. You take a sample (large enough for at least one gravity reading) from the fermentor after you’ve aerated and pitched the yeast, and incubate it it warm conditions and with frequent shaking (preferably on a stir plate or shaker) to force it to attenuate fully. We’re not really worried about how it tastes, so we can use high temperatures and stirring, but again you need a sterile flask and to take the sample aseptically, as any contamination could change the results. Once you know the maximum possible attenuation, you are better placed to make decisions how fermentation is progressing and to diagnose any problems with over- or under-attenuation. It should finish fermenting well before the main batch, so you can use the information to decide things like when to raise the temperature for a diacetyl rest, or when to rack for conditioning.