Watching how well Karen did the mount, he saw her pause.  He places a firm hand at the back of the ankle closest to him to show her he would not let her fall.  The other hand was at the back of the saddle, but not touching to her, just ready if she needed him to grab her. 

"You did great."  He looked up at her with a look of encouragement and pride at her efforts.  He reached over for the reigns and handed them to her. 

"Once you feel secure there, with both feet staying in the stirrups, give Dora a light tap with a bringing inward to her belly with your feet.  Don't kick, unless you want her to go fast right on the go."  He cautioned firmly but kindly. 

He had an image of him running after the two of them trying to catch up to save her, it could be funny but it could also go very badly.  He thought back to how Cressida had died as they raced in an unfamiliar area.  She had been looking back to see how far away he was and teasing her when her horse had been startled by a snake and taken away from the path they had been on and she had hit a tree branch that had knocked her off the horse.  Then she had hit her head on a rock.  It had been nearly instant.  He had only been able to hold her and ask her not to go before she closed her eyes with a soft look up at him as she died.