I hate to get serious about this photo, it is too much fun!
The only times I have seen dogs like this on a dog sled is when they were overjoyed at the racing, rolled over and squiggled in the snow. Most yelp and leap in anticipation or actually during the start, after the order "Mush!" That shows things down, of course. Then the one dog decided it had to take a dump, it was so excited.