Someone with the “upper hand” has the final say over a situation.
When a group of youngsters gather to pick sides at a game of sandlot baseball, the two captains decide who chooses first when one of them grasps the bat at the bottom of the handle.
The captains take turns gripping the bat one fist over the other until there was no more room.
The last one to fully grip the bat handle has first choice, and he has the upper hand.
We often say that someone in control has “the upper hand”.