That is absolutely a wonderful point Bob!
When I exercise these 'non-active' drills like supermanglide, I always try to focus on shaping my vessel as straight as possible and trying to reach the other side of the pool in as few glides as possible.
Never have I focussed on total relaxation or even approximating a going to sleep feeling!!
Thanks a lot Bob: next pool sessions I will put all my focus on this relaxation and loosen the dropping the SPL target