She struck Ron hard across the head with her stone arm

Ron started to direct the black [chess] pieces. They moved silently wherever he sent them. Harry's knees were trembling. What if they lost?
"Harry -- move diagonally four squares to the right." Their first real shock came when their other knight was taken. The white queen smashed him to the floor and dragged him off the board, where he lay quite still, facedown.
"Had to let that happen," said Ron, looking shaken. "Leaves you free to take that bishop, Hermione, go on."
Every time one of their men was lost, the white pieces showed no mercy. Soon there was a huddle of limp black players slumped along the wall. Twice, Ron only just noticed in time that Harry and Hermione were in danger. He himself darted around the board, taking almost as many white pieces as they had lost black ones.
"We're nearly there," he muttered suddenly. "Let me think - let me think..."
The white queen turned her blank face toward him.
"Yes..." said Ron softly, "It's the only way... I've got to be taken."
"NO!” Harry and Hermione shouted. "That's chess!" snapped Ron. "You've got to make some sacrifices! I take one step forward and she'll take me -- that leaves you free to checkmate the king, Harry!"
"But --" "Do you want to stop Snape or not?"
"Ron --" "Look, if you don't hurry up, he'll already have the Stone!"
There was no alternative.
"Ready?" Ron called, his face pale but determined. "Here I go - now, don't hang around once you've won."
He stepped forward, and the white queen pounced. She struck Ron hard across the head with her stone arm, and he crashed to the floor - Hermione screamed but stayed on her square - the white queen dragged Ron to one side. He looked as if he'd been knocked out.
Shaking, Harry moved three spaces to the left.

**
What’s the meaning of the highlighted? (She struck Ron hard across the head)