As Carol and Ontario Sheltie posted, there is a correct way of using a furminator on a Sheltie.
Used probably, particularly when you've a pup that is blowing coat (or in Indy's case "hoarding" coat), the furminator can be your best friend.
The key is is to NEVER follow the instructions from the manual!
Use the Furminiator as a line brush, focussing on the undercoat only. Never use on the top coat. For Shelties with really big pants, it can be a god's send.
I wish I had taken the furminator to Indy before his last grooming. That might have avoided the big haircut that the groomer did on him.