I wondered at one point if they'd tried to set it up so if you put in one word it worked for all of the related words (adding s/ed/ing/ation etc) and took the highest position of all of them. But then some words you could put in multiple versions.
However I think it would work better if you didn't have to put in multiple versions. On semantle you tend to find if (eg) walk is on the greens then walked/walking are also greens and you can use which is higher to work out what part of speech it's closest to. With clemantle sometimes you'll find one is high 900s and the other two are very cold, but that means that when you think of a word you have to try all derivations to be certain that is isn't helpful.