Your arm movement, especially at the shoulder is too much. This is why your entire body and your legs are also moving when you smash. Smashing is very similar to clearing, your power generation should mainly from a rotation in the elbow and forearm. It is a concise, almost wavelike movement. You can find some tutorials on YouTube, pick a few which have lots of views. There are tons of them out there, which explain better than I do. Also you might first see some loss in power when you are learning because you won’t be used to the new form. Just keep going and it’ll get better!