I immediately had to think of our Siberian Husky, who loves doing that, too.
For example, during summer he loves lieing down in some water or on marble surfaces. The increased surface contact clearly helps cooling off:
(He has his fourth leg below his belly since he's lacking space in there.)
As far as I'm aware there's no special meaning attached, unless his behind is still in the air (= "Play with me!"):
As long as he's not showing any other behavior (like being nervous, doesn't let you close, etc.) I wouldn't worry about it and it's mostly just comfortable in some way or another.
Since you also mentioned trimming the fur: This might help, but it might actually hurt the dog as well. This heavily depends on the actual race as well as his environment (like temperatures).