Fit For Golf offers a series of exercises to work on internal hip rotation. Hip rotation is important, obviously, for clubhead speed.
Perhaps just as importantly, proper hip mobility is key to avoiding lower back injury. Mike Carroll of Fit for golf writes:
lack of internal hip rotation can place more stress on our lower back when we swing. We need to rotate to make a decent golf swing. If we cannot rotate from our hips, we may search for rotation from our lower back. This is not a great strategy, as the lower back isn’t really designed well for rotation. If we can loosen out our hips and rotate predominantly from there instead, it may alleviate some lower back stress. If you find your low back aches from golf, IHR is definitely something I recommend you check, and work on if necessary.
You should check out Fit For Golf’s videos. I’ve found them useful.