Hi Kimberly
Let’s start at the beginning: what is your intention? Why would you want someone to be in this pose? Depending upon your answer (your intention) you may want the legs engaged (the quadriceps specifically) but you also may want to just let everything soften. In the yang world this pose is known as upavistakonasana and it is often taught with the leg muscles engaged, the knees and feet pointing straight up and the spine straight. This creates a very deep external rotation for the hip sockets, but it is a very muscular pose. In Yin Yoga we try to go past the muscles, so we relax as much as we can. As your teacher said, “let the legs go where they go!” That also applies to the feet too. Why are you concerned about that?
If your intention in Dragonfly (aka Straddle) is to stress the adductors, and to a lesser degree the hamstrings, then we would want the leg muscles relaxed so the stress can soak into the fascia of these muscle groups. There is no need to engage the muscles as that will take stress off the fascia. (See this
explanation as to why we want to relax muscles in Yin Yoga postures.) However, for some people, the stress of this pose can tug too much on the inner ligaments of the knee (the medial collateral ligaments.) If pain arises here (and anything sharp or burning would be considered pain, but dull achy sensations are okay) then the student can either engage the quadriceps to tighten the knees and reduce the stress ont he ligaments or simply narrow the width of the legs. Personally I prefer to narrow the legs slightly because engaging the muscles for the whole length of time that you are in the pose can be problematic.
In general, in Yin Yoga we do “turn off the muscles” so that the stress of the pose soaks deeper. But, never is never right and there may be times for certain students to keep some engagement. Remember you intention and have the student pay attention.
I hope this answers your question
Cheers
Bernie