Getting a good sound when you strum a guitar is all about how you hit it.
The more your arm swings when strumming, the more force you apply. Strumming with a straight wrist and forearm creates the most power, but without some finer control it can sound brash and stiff. Relax the wrist and forearm, and allow the wrist to “snap” on the downstroke. It’s no different than swinging a baseball bat or a golf club…power comes more from movement than muscle.
Experiment with how striking the strings differently creates different sounds. If you’re used to pumping the arm back and forth, it’s very possible that you might be holding tension in your wrist and hand. You might also be hitting the strings too hard. Pay attention to what you feel and see what happens when you lighten up…and don’t forget to check in with your neck and shoulders as well! It’s easy to hold a lot of tension there and limit both range of motion and control of the hand,
On the flipside, you might be strumming with the wrist alone, moving the hand from side to side but keeping the arm still.
This not only overworks the muscles of the wrist, it limits your dynamic range. Remember, big sound comes from the way you move more than how hard you hit. A flexible wrist lets you play more lightly and smoothy, but the wrist alone won’t create enough force to make the guitar really ring out.
As you explore, just consider that the strumming hand has two basic ranges of motion: back and forth (or up and down, if you’d rather think of it that way) or in and out (a rotation of the forearm). Back and forth gives you power, and rotation gives you subtlety and more speed. Good strumming technique is a combination of both.
Start to pay attention to your own strumming habits.
Don’t worry about making changes yet…start off just observing. If you find tension, just let it relax and see what happens. Remember that there’s basic rules of mechanics at work here: equal parts art and science. When your hand moves more naturally, you get a better sound and more musical flexibility. So spend some time learning to distinguish “habitual” from “natural”. You might be surprised just how much your musicality and skill can grow simply by paying attention to details you might not have noticed before.