The keeper can move. The penalty would be repeated if he doesn't have one foot on the line. There's plenty of GKs that start their jump movement before the ball

The keeper can move. The penalty would be repeated if he doesn't have one foot on the line. There's plenty of GKs that start their jump movement before the ball is kicked. example: https://www.youtube.com/watch?v=JNQ6XdrEhqc https://www.youtube.com/watch?v=JNQ6XdrEhqc edit, better example: https://www.youtube.com/watch?v=I4EtbA2Nlyw https://www.youtube.com/watch?v=I4EtbA2Nlyw