Maddie DeHart and her connemarra gelding Thor are this week's pic. Maddie is showing an excellent half-seat position. Half-seat is used for cantering, hills, in between jumps of a combination (as seen here), etc. It is a lightening of the seat without getting all the way into the two-point, or jumping, position.
Maddie's knees and thighs have contact with the saddle, while her seat comes slightly out of the seat of the saddle. She has a great straight back and has her eyes up. She also has a good connection with Thor's bit. She has brought her hands back from the crest release so she can influence him in between jumps.