Dude, she didn’t even need to grab the reins if she wanted to go fast; she just had to wait for three seconds, until Fenris realized he was close to the spot where he saw a kid/cat/dog that one time, and bolted.

Transcript:

Mr Bugg is sitting on Fenris’ back, holding onto the hair with one hand and waving to get Mrs Bugg’s attention. He is looking a bit worried. Mrs Bugg is still standing, but she has grabbed the leash/reins and is probably screaming at Fenris to go faster, encouragement he does not require.

The text reads: “Mrs Bugg has grabbed the reins on Fenris … now it’s on.”