What is a release word? It is something that tells the dog its ok to stop what they are doing. It is different then the positive or negative marker. The marker is designed to tell the dog if they are on the right or wrong track. In the game of hot and cold it is the "your getting warm" or "your getting cold." The release word is used to signal that an exercise has been completed and the dog is now free. It needs to be different then the markers.
Release words are most often used with stays. You don't want to tell your dog to stay and have them decide when its time to get up. You want them to stay until you tell them its time that they can get up. You would use your positive marker to re-enforce your dog is doing the right thing. For example, when I am first teaching a dog to stay for a longer time, I will make many return trips and use many positive markers along with intermittent rewards to tell them that are doing good and on the right track. I then use a release word when we've reached the amount of time we were trying for to signal it is ok to get up and move again. Little by little the dog learns they must do the stay (or whatever other behavior we are training) until I give them the release word to signal they have finished the exercise.
It is important to pick a release word that you do not normally use in normal conversation. With my first dog, I made the mistake of using the word ok. I would be in conversations across the room and every time I said ok in the conversation my dog would jump up assuming her job was done. All the following dogs I have been careful to pick a word that I would never use in conversation.