I think the differnce is when whistling your tongue is totally flat down (on lower notes). But when you play, the middle of the tongue is placed forward in position of the attack (tip of tongue behind bottom teeth). And if Im right the sides of the tongue stay there (in all registers?). This position of the tongue also helps the corners of the lips together (and make the chin flat). It also make the corners not smile or (down sour). Just firm together – not straining. Greg had a good word for that instead of tension – engaging??? or supporting.

Am I right about the tongue position?