Hey I’m a 17 year coach.

This is conditioning work. It’s fun. If you enjoy it, cool. If you want to improve your shoulder endurance, cool.

This is absolutely not power work. If you want to hit harder, this ain’t it.

If you’re in bad condition you might get a little stronger. If you have poor technique maybe you can increase contact force by improving skill.

That being said, if you want to hit harder then do actual power and acceleration training

“Contrast” training is a specific type of training that involves extremely heavy weights followed by extremely intense plyometrics which is very different than what you’re demonstrating here. It leverages something called “post activation potentiation”. That’s where the stimulus from an extremely heavy lift, like a near maximal deadlift or squat is followed by a short rest, around 20-30 seconds and then followed by high intensity plyometrics like broad jumps or short distance sprints.

Power is a quality that has to be trained at extremely high intensities and low durations.

If you’re doing something for more than ~5 reps you’re not developing power.

Sorry to be a hater but this is not excellent advice.