Exrx and other sources claim that flutter kicks are a good psoas exercise. My psoas are tight, and I use a tennis ball to loosen them up. When I do flutter kicks, I feel the workout in my hip flexors in the inner thigh, and in my lower back. But nothing in psoas. Is there a way I can ensure that the flutter kicks work the psoas, instead of the other (presumably compensating) muscles taking over?
The psoas is one of the hip flexors. This is a group of muscles that either raise the legs up towards the head, or pull the body towards the feet depending on which point you anchor.
Flutter kicks do work the psoas along with the others, but in a limited fashion as they won't be going through the full range of motion.
If you want a better (in my opinion) exercise to work the flexors, lay down on the ground on your back. Put your hands over your head and grab an immovable object (I use a situp bar that goes under a door), and while keeping your legs straight, raise them up to a 90 angle. (Touch your toes to the door/piece of equipment if you are using that.) Lower them in a controlled fashion back to the floor.
Once you get better at the motion, you can increase the exercise by pushing your feet straight up at the ceiling once you get them to the 90 degree angle. This will help work your lower back extensors and your abdominal muscles, but it will also increase the strain on your spine, as you kind of "roll up" onto the backs of your shoulders. If you have back/neck pain, I wouldn't recommend the addition.