There was science and a rare knowledge of leverage in the mild gesture.

When a man is kicking, he is on only one foot.

And, the right sort of oblique push will not only throw him off his balance, but in such a direction that his second foot cannot come to earth in position to help him restore that balance.
Under the skillfully gentle impact of Brice's shove, the man let go of the snarling collie and hopped insanely for a second or so, with arms outflung.

Then he sat down ungracefully on the sand.
Scarce had he touched ground when he was up.
But the moment had sufficed for the collie to go free.
Instead of running off, the dog moved over to Brice, thrust his cool muzzle into the man's hand, and, with wagging tail, looked up lovingly at him.
A collie has brains beyond most dogs.

And this collie recognized that the pleasant-voiced, indolent-looking stranger had just rescued him from a captor who had been treating him abominably.