Defender at the “top of the box” is shielding the ball, not having touched it. They are hassled by an attacker. The defender calls and the keeper comes and gathers the ball on the ground. The keeper calls “keeper” as they arrive.

This drill simulates a typical situation in a game, and helps build cohesion between defender and keeper. The keeper learns to be vocal, and to judge the danger from coming for a ball which is shielded by the defender.
Tips for coaches – Confidence Cohesion Technique
Any coach can utilize these drills with their keeper. The objectives are the same – enhance footwork, follow the ball better with the eyes, and judge the flight of the ball.
A coach can use any one of the drills independently, and involve multiple players, especially attackers, to sharpen their skills too.
Try to encourage the keeper to use these skills – their footwork, their eyes and reflexes, and be brave to make the judgement calls and decisions that will come up regularly in games.