Just a thought ...
...I find if I contract my stomach muscles a little more (feeling as if I'm approaching arching my back) and a little earlier, then it's possible to then relax them at the right moment - when the mouth gets to air and the glide is at max.
p.s @Grant: nice to read your description as it exactly matches what I posted a while back but got no response to :)
A psychological disorder is: "Any personal construction which is used repeatedly in spite of consistent invalidation."
~ George Kelly
"The water is your friend.....you don't have to fight with water, just share the same spirit as the water, and it will help you move."
~ Aleksandr Popov