There are different types of seizure service dogs; seizure alert, seizure response, and seizure assistance. The role of the seizure alert dog is to detect the impending seizure and signal the owner. Hopefully, this will give the person enough time to either take anti-seizure medication, or at least get to a place that minimizes potential dangers. This of course requires a strong relationship between the human and dog. They need to be able to read each others' body language. The dog has to detect the subtle changes their owner might display before a seizure, meanwhile the human needs to understand when their dog is trying to warn them. This partnership could take time to develop and it is thought that over time, some dogs have been able to detect seizures using their sense of smell.
Here's one young girl's story about her relationship with her seizure alert dog.
A seizure response or seizure assistance dog is taught to react when the owner is experiencing a seizure. The dogs are usually trained to stand by, or over their owner throughout the seizure. This ensures that the person will be kept safe during the seizure and it allows them to see a familiar face once the seizure is over. The dogs can be taught to retrieve and/or use a pre-programmed phone to call for help if their owner needs it. Many of the stories I was reading about involved children with seizure assistance dogs, so the phone wouldn't necessarily be as important since most of the children would have an adult within earshot of warning barks. Also, seizure service dogs wear a harness with pockets where medication, a phone, and medical information can be stored so if an adult does have a seizure out in public, a passerby could convey that information to emergency medical services as needed.
An older girl shows how her alert dog is "on-duty" even as she's getting ready for bed.
Last January, a school barred a boy's seizure alert dog, but then the officials overturned their decision after his story got media attention. I don't know if the boy ended up going back to school or not, but it is a shame that he was put in the position to choose between his education and his safety. His seizure assistance dog actually had a magnet on her collar that she was trained to swipe over a nerve simulator in the boy's chest when he was having (or about to have) a seizure. This would lessen the effects of his seizure.
If you'd like to know more about seizure service dogs, the Delta Society has a great pdf document that talks about different breeds, how the dogs help, and where to get one, among other things.