l could only power in the summer but l wanna do it in the winter how can l make it happen?

Well, it is kinda hard to play handball in the winter because it is really cold and your hands can be numb. When l play handball in the winter l usually wear my gloves and play with ace ball instead because it is impossible for me to play big blue ball with gloves on. If you are playing with gloves, put a glove liner on so it does not hurt when hitting the ball

strengthen your arms

If you want to kill (or possibly roll) just practice on a empty court for a while aiming only where the wall meets the floor hitting a shot that is like 1inch off the ground (idk) and after awhile you wo not have to think about hitting a killer. this also helps increase power

you should wear hand warmers before playing and wear some extra padded gloves if you want to play in the winter