Hip rotation refers to the movement of the hips or pelvis in a circular or twisting motion. It is when your hips turn or rotate either towards the front or back of your body. Full definition
Did you know that a lack of hip rotation is a huge contributing factor for lower back injury? (onnit.com)
Watch your form as you switch legs - we all have a dominant side and sometimes even a subtle hip rotation can cause some variation in your form between each leg. (thebettyrocker.com)
Indeed, when comparing the clam exercise without added elastic resistance with the side - lying hip abduction and the side - lying hip abduction with external hip rotation, McBeth et al. (2012) reported that the clam exercise caused the greatest activation of the anterior hip flexors and very gluteus maximus EMG amplitude. (strengthandconditioningresearch.com)