The first time, they encounter a threat big enough to have them have to flee to regroup, Edwin freezes.
Charles recognizes the response immediately and knows that if he doesn't force Edwin to move, he won't. So he takes hold of his arm, spins him bodily, releases him and presses his palm against the small of Edwins back, and forces him forward, away from danger.
Once Edwin is in motion, he runs like he was born to do it, but often, he freezes first, and Charles has to make him move.
Edwin because it's the lingering response born out of decades in hell, freezing to not draw attention to himself, running when forced to.
He tried to apologize in the beginning, and Charles would always rebuff it.
"It's alright" and he meant it every time.
Charles doesn't talk about it either. Not just out of respect for Edwin's wish not to talk about it, but because he recognized the response from his own home.
Any time his father raged, his mother would freeze, standing so so still as to not drag attention to herself, crying quietly and unable to move. She wouldn't unfreeze until the violence was over, softly treading towards Charles to look him over and apologize again and again.
"It's alright," he'd say, and it always tasted bitter on his tongue, but he knows she couldn't react in any other way unless forced to move.
So he forces Edwin to move.