Q: I was told I should lengthen my IT band in order to not overuse my quads in tendu. How do I do that?
A: The simple answer is: You can't lengthen your IT band. The IT band is composed of fascia, which is dense connective tissue. It's wrong to state that it can lengthen like a muscle. Fascia can't create movement; it can only allow or restrict movement depending on its relationship and balance with the surrounding tissues.
If you have tension in any area along a fascial line, it can influence the muscles that connect to it. A chronically tight IT band may be an indicator that other lateral hip muscles (usually the gluteus medius) aren't doing their fair share of the work. The fascial line that more directly influences the quads is called the superficial front line.
As far as a tendu goes, the IT band and the quads have totally different roles. You must engage the quadriceps in order to tendu front and side, while the IT band acts to stabilize the lateral knee and to coordinate the muscle action of the glutes.
Perhaps your teacher was using an image of the IT band lengthening to keep the tendu tracking correctly, but it might be better to think of sending energy through the center of the hip, knee and ankle joints instead. Hope this helps to "de-myth" this correction!