I thought that picture of Porter was interesting as it's more unusual for a sheltie to sit like that as opposed to bigger and heavier breeds. Certainly anything the size of a lab upwards tend to sit either backwards (the 'rock sit' as Dr Shelli has described) or sit on one hip, which is frequently described as a puppy sit. I have found that shelties (my own and others) tend to naturally sit correctly balanced, which only leaves the position in relation to the handler and the duration to work on.
Slightly off topic but I have always taught the dogs to down from a stand (dropping forehand first, then hindquarters) and never to down from a sit as it encourages them to slide down during a stay and also forward movement which is penalised in Distant Control.
Slightly off topic but I have always taught the dogs to down from a stand (dropping forehand first, then hindquarters) and never to down from a sit as it encourages them to slide down during a stay and also forward movement which is penalised in Distant Control.

Like the dog isn't ready to work or move. I like them up, weight on the front legs, ready to power out of that sit or comfortable enough to hold that sit until I release them. That is the agility angle here. Seee. LOL 
