It's by Helen Sword, and seems to be something she set up to help promote her book, The Writer's Diet. You cut and paste up to about a thousand words of writing into the box, hit 'Run the test' and within a few moments you get a fitness report, telling you if you might have overegged the literary pudding with too many 'be' verbs; abstract nouns; prepositions etc.
It's a lot of fun. I tested my latest thousand-word flash, A LITTLE RETRIBUTION FROM BEYOND THE GRAVE, on it and was pleased to find that I'm a lean mean writing machine...type...thing.
Now, if I could only get the Wii Fit to agree, I'd be well chuffed.
If you decide to give it a go, let me know how you get on.