You make is sound like your left thumb is tucked inside your closed fist... in my experience, that's a mighty fine way to break your thumb .__.
Okay, so that was pretty good. For battles, you wont need to include such fine detail as what your thumbs are doing. Just stating whether or not you are using an open or closed fist would be enough.
Also, your punch reads to me that you are just going through the motion of a punch, as if this were a dojo and I were to be schooling you on technique and drilling it into your muscle memory. The power from a punch or a kick, starts from an anchor point of which it draws the power from. In this case, your feet are your anchors and allow you to push into the ground as you make your punch, to give you a stable platform to punch from.
Stances help you gain adequate power for you attacks; while yes, a basic position of feet side-by-side and shoudler-width apart does allow you to push sideways into the ground to gain your power, it leaves you in the most open silhouette for counter attacks and sweeps.
A stance with one leg slightly behind you (both feet still shoulder width apart) allows you to push off your back foot to deliver more power, with the front foot being the main anchor point to catch yourself after you finish your punch. You can also decide to thrust your weight forward off your back foot and then push through your front foot to gain even more power, but leave you in a more vulnerable position as you are forced to follow through with your weight. This stance also allows you more versatility in the way you counter, having the option for kicks, punches or to step away or into your opponent.
Anyway... does that make sense?