Supta Buddha Konasana or Reclined Butterfly Pose (Core/pelvic floor stabilization, closing bent legs, slowly, supine 'butterfly' pose)

Lie on your back and bend your knees.
Put your feet together and let your knees drop apart.
Bring your feet a couple inches apart.  
Slowly engage the pelvic floor and transversus abdominus (deep core) muscle, by pressing your lower back into the mat and then engaging inner thigh muscles very slowly.
Then bring the knees all the way up very slowly.
Your muscles might shake at this point but that is completely natural.