As both instructors and students, we hear the phrase "hip opener" thrown around quite frequently in yoga classes. But what exactly are we doing when we state that we are opening the hips?
To understand hip openers you first have to understand the anatomy and mechanics of the hip. The hip joint (or femoral acetabular joint) is a ball and socket joint. The ball is known as the femoral head and is attached to the femur (the thigh bone). The socket is called the acetabulum and is a part of your pelvic bone.
Because we are weight bearing creatures on our legs, our hip joint needs to be a very stable joint. We wouldn't want it to risk dislocation from daily activities. That means the socket needs to be deep, providing a lot of surface area contact and bony stability to the ball and socket joint. The trade off for this stability is a lack of mobility. Read: the hip joint has much less range of motion than it's upper body counterpart - the shoulder.
The hip joint (and all ball and socket joints) move in combinations of 6 primary motions: flexion, extension, abduction, adduction, internal rotation, and external rotation. Hip flexion is bringing the knee closer to the chest. Hip extension is bringing the femur posteriorly (past the body). Hip abduction is moving the leg outside (think making a snow angel). Hip adduction is bringing the femur back towards the midline. Hip internal rotation is rotating the femur inward (towards the middle). And hip external rotation is rotating the femur outward. Some combination of these motions creates all movement we see and feel in the hip.
When we "open" the hip we are creating a large range of motion in one or more of these directions through a combination of stretching muscles, mobilizing the joint, and mobilizing the capsule (the saran wrap like covering around every joint).
Therefore, a "hip opener" can be any posture that creates a larger range of motion in ANY of these directions. However, when we hear (or say) that we are working on "hip openers" we are typically referring only to positions that bring the hip into external rotation (ex. half pigeon, full pigeon).
This doesn't mean we need to stop doing these hip openers, instead just become more cognizant and purposeful of how we speak and what we truly mean.