Tough call. Many coaches, and even some pitching coaches believe that forced hip rotation adds to power. If you watch the top college, international and professional pitchers in slow motion, only a handful close their hips at release. Everyone else releases with the hips at about 45 degrees, then follow through naturally, which at regular speed looks like they are closing sooner and forcefully. Here's an article from Michele Smith that talks about this:
www.michelesmith.com/2007sbmissue2.pdf

When you're rotating your hips you will need the strength of your inner thigh and a strong core.

I think your coach just wants you to transfer all your weight to your front foot when you hit. he wants you to get your power from your hip rotation. Explode as your rotate your hip and transfer all your weight toward your front leg and keep front leg extended.

http://www.softballperformance.com/softball-hitting/key-concepts-of-hitting.html

Sometimes You can get these bungee things that go around your waist. It will give you resistance so you have work harder. Just keep practicing. Which coach told you this anyway? We haven't had practice in a while

Thanks everyone for your answers!
KC, Coach David told me that at the practice when only me, Rach, and Sierra showed up. I just want to approve on that for when I play for the school again.

swing a bat in the pool. work on you body core and rotational flexability and strength. multidirectional squats and lunges.

I sorta have the same problem but i am now a lefty so i slap and drag. Good luck though.

idk!!!