Static Spear Thrust.
Now that you have identified your psoas, it's time to use it in the spear thrust. You can use a typical 6 foot hardwood staff as your training implement. When you take the spear and sit down into the back leg to prepare the thrust, you should feel the psoas stretch in the front leg. Specifically, you should feel the psoas connecting the spine and the femur. The deeper you sit and the more you twist to be sidways, the more you should feel this.
Now, you will extend the spear forward. Push off the ground with the back leg, and allow your body to be twisted to face forward. Your pelvis will tilt forward-- this is functionally the same position that is held in the Aunkai stillness training. As you extend your arms in the spear thrust, and pivot to face the front, you will want to feel the psoas tighten on the back leg. Once you have fully extended the spear, you should feel the weigh of the spear actually stressing the psoas on the back leg, specifically toward the top of the femur. You will want to feel the back leg connect to the front hand, and the front leg connect to the back hand. You will want to lift the head and feel the tail bone pulling downwards, even at the full extended position. There is some other stuff going on with the cross (upper chest/shoulder girdle) as well but I'm going to leave that out for now. If you've been doing a lot of jujiko then you can play with that awareness during the thrust as well.
Depending on how much conditioning you already have in your body, you may also feel like your skin is stretching as a result of the weight of the staff. You can try to feel like the entire back of your body, from your heels, up the hamstrings to the top of your head, is being stretched by the bo.
Once you feel like you have hung out long enough, just retract the staff. You can do this by sitting down into your stance, and then allowing your torso to rotate as a result of your pelvis rotating back towards the rear leg. By rotation, I mean rotating with the spine as an axis. If the rear leg psoas is tightened by your thrust, then you will naturally twist back towards the leg anyways. As you twist back and retract the spear, you will tighten the psoas on the front leg again.
Can you give a bit more details here? I'm not able to visualize the body doing this.
From the very start, where are your feet pointed? Where are your hips pointed? Where is your face pointed? Are all three in align? Whole body facing forward with the spear? Or are you facing one direction and the spear is 90 degrees from that?
You say that when extending the spear, "allow your body to be twisted to face forward", so I'm guessing that you aren't starting with the body aligned with the spear? So, what position is the body in when you start?
Do the feet move? Do the hips turn? How?
I just can't get myself to visualize what you're trying to do.