This is what the description said: "The patented design utilizes sensor technology to detect nail differences between the live blood of your pet's quick, and the dead skin of the nail and claw."
It's also supposed to be for dogs up to 75 lbs and both of my black nailed dogs are over that but maybe there's no sensor for weight.
The medium size one will be out in August and it sounds like they will have a small/cat one, also. Some one-line testimonials I read just rave but there was also a long review on Petsmart that said they were crap, didn't work on her 50lb dog, the blade is dull and can't be replaced or sharpened and she's taking it back. So.....I'm hoping to hear from someone on here that has tried it, good or bad.