In my very first ballet class I was told that a plié is when you bend and straighten your knees. That's it. That's all the information I was given. While that technically isn't wrong, it isn't complete information either. If you are standing with your weight on your own two feet and you bend your knees you have two options: you can bend other joints as part of the plié or you can fall over.
Since our students are not usually falling over as they plié, it is safe to assume that they are bending in other joints as well and just don't realize it. Given that "bend the knees" has worked so well for so long, why add more information now? Because it hasn't worked so well. Better information will lead to better pliés. All that gripping you see your students doing? The hip flexor pain they complain of even though you are intentionally keeping their extension low while the pain persists? Those shallow pliés that make no sense because you've seen your students stretch their calf muscles and you know they have more flexibility? All of that is caused or contributed to when students misunderstand plié. (The hip flexor thing should probably involve qualified medical advice if it persists. I'm not a doctor or PT.)
Ballet students are incredibly hard workers and they put a lot of effort into doing exactly as instructed. So when we teach plié as bending and straightening the knees, students will do their very best to only bend and straighten their knees because that's exactly what we told them to do. They grip in their hip flexors and hip extensors trying to keep the pelvis from moving "too much." They grip their toes on the ground to try to keep the ankles stable (and/or because they aren't engaging their rotator muscles to turn out). They want so badly to keep their body stacked up exactly they way they understand ballet "should" be.
But dance is movement. When I think of the word "movement" I think of freedom, of a paint-soaked brush making broad strokes across a page. When I think "bend your knees" I think of a squeaky door hinge. How can we make plié a beautiful, tension-free movement (even though the knee joint actually is a hinge)? It starts with bending in three joints instead of one.
As the knee bends, the hip and ankle also bend to balance the change and maintain the body's most useful alignment. (If you are unfamiliar with the idea of alignment being constantly changing, I highly recommend that you look into Eric Franklin's work.) When we don't try to consciously override this process, it happens very naturally and the body instinctively finds the appropriate amount of flexion to maintain function. But beware! Just because this is a natural response doesn't mean that our words as teachers can't affect it. If you have only talked about plié as a knee movement, your students have subconsciously restricted movement in their hips and ankles. They need permission and guidance to release that tension and really get the most from their pliés.
Talk with your students about the three joints moving as a team. (Some people prefer to talk about 6 joints since plié involves both legs. 2 hips + 2 knees + 2 ankles = 6.) When this is a new concept, prompt students to notice what their clothing/skin (depending on your dress code) is doing during plié. When all the joints are involved in a healthy way, the tights stretch over the knees and fold/wrinkle at the hips and ankles. Encourage students to imaging that their tights need to fold just as much as they stretch and see what the result is.
When your students are ready to take the concept a little further, add another stretchy part to the plié. To get the best ankle flexion, students need to release tension in the front of the ankle joint. One of the most effective ways to trigger this is to have students imagining their feet stretching as they plié. There are lots of images that work here: toes stretching away from the heel, the bottom of the foot widening like a pat of butter melting in a pan . . .
Tension in the hips can be harder to release, partly because the gluteal muscles are hip extensors and they are strong! And the glutes don't usually completely release during a plié. They are just too useful for things like stabilizing the turnout that the deep six external rotator muscles initiated. It is often more effective to encourage students to soften the front of their hips as they plié. Start by having them place their fingers on their hip flexors and just notice what it feels like during plié. Feeling what their body is actually doing helps students transform information from theory into reality. Now that you have this baseline, explore different ways to relax the tension (imagery, somatic approaches, working in untraditional shapes, etc.). When students are in a less rigid condition, physically and mentally, try the pliés again and see if they feel different.
Plié as a movement in three (or six) joints is a concept that I explore with all of my students but we really dig into it on the deepest level in Level 5 in the Intermediate Curriculum. Why? Because the key principle in Level 5 is turnout. Of course my students have been using their turnout for years by the time they arrive in Level 5 but this is when we are ready to focus on and refine the concept. (A bonus benefit is that this dovetails really well with their pointe curriculum too, so we are reinforcing concepts across different classes.) In Level 5 I like to have students initiate plié by melting their baby toes into the ground and starting the movement in the hips. The knees still travel away from the midline, so that's what the audience notices, but the muscular engagement is far more efficient and serves the dancer's technique much better.
Try it in your classes and let me know how it goes. I'm confident that you'll see improvement.