I think the flea control that's in Sentinel is the one that has the flea "birth control" in it... (on it's own it's called "Program"). If it's what I am thinking of it doesn't actually kill the fleas, it just keeps them from breeding if they bite your dog. There is a substance in the medicine that inhibits the growth of chitin, which is what forms the exoskeleton of the flea. When chitin can't develop, fleas are too weak to live.
The vet I used to work for always recommended giving that year-round. Back then they didn't have the combination, he recommended Interceptor and Program together all year-round. I am almost positive that's exactly what Sentinel is- Interceptor and Program combined. (If I'm wrong, someone please tell me so I don't look like an idiot too much longer!)