What makes a plié different from just a knee bend? For the most part, the alignment of the body.
A squat has a knee bend, but the alignment of the torso and pelvis are very different. As the knees bend, the hips go back behind the feet and ankles. The torso angles into more of a folding over position in reference to the thighs. Weight is transferred to be predominately in the heels.
I'm a big fan of squats. They are a great exercise for the glutes, quads and , if done wisely, the back. They are a very functional movement for human bio mechanics. But if you try to do a pirouette from a squat you will learn some unwelcome lessons about basic physics.
As well as making certain dance steps possible, plies, if done properly, offer a great lengthening component to building strength in the muscles of the legs. The muscles of the core are also engaged through the execution of a plié.
I have often found that focusing on the placement of the pelvis can aide in the proper alignment of a plié.
Towards the bottom of this post is a short video tutorial. You can scroll down, or hang out for the pictorial tutorial first.
Toes pointing forward
Bend knees, weight is evenly displaced down through each foot
Pull belly button towards spine and imagine that the back of the pelvis is sliding up and down an imaginary wall
Shifting the weight to the heels, open up the feet as you straighten the legs.
Evenly displace the weight through the feet again
You are now in first position.
Feel the glutes "wrap around" and gently squeeze
As the knees bend, imagine the back of the pelvis is sliding up and down an imaginary wall.
Knees stay opened out over the tops of the feet.
As you straighten the knees, feel those glutes do that wrap and squeeze.
Here's a short tutorial video-
Hope this helps.
Thanks for watching!
Enjoy the dance that is life!