It's a power move. Power hungry bureaucrats often use this move to display a sense of dominance. Trump & Putin (sometimes) also like to use this move. Esp, Trump if he's on the left. He will outstretch to "enclose" the other party
Trump likes to play this game and plays it well. Even when he's on the right (power position), he offered his hand, palm upwards.
So when the photo is taken, it looks like he's "leading" Putin like a little girl.
Hand over the shaking hand. It can be interpreted as being friendly or dominance. Depending on case.