What is the purpose of prisms in vision therapy?

It's been a week since I'd started using a seven diopter prism, base-in, with the fixation cards exercise, and I've been noticing quite a difference.  In just a week, I'd made quite a lot of progress toward fusion.  Still not yet there, but getting closer and closer.

I am now able to sort of fix with both eyes simultaneously while doing the fixation cards exercise.  It's a little weird because I don't yet have full simultaneous control of both eyes, so there is this sort of confusing feeling of my head existing in two separate realities.  Like my head is in two different rooms, and one of the rooms is drifting around a bit.  But overall it's a nice sensation, because it feels like I am doing the right thing, and this is where I need to be going.  So I'm now fixing, but not quite yet fusing.

So what is the purpose of prisms in vision therapy?  I think a not bad explanation is that they are like a stepping stool.  They make it easier to achieve fusion in the first place, by accommodating for whatever eye turn you may have.  In my own experience, I think an even better explanation for what prisms are in vision therapy is that they are like sticky hands.

Imagine that you are in a big, empty white room.  Except that in the middle of it there is a big white cube, and for some inexplicable reason you feel an incredible desire to get on top of it.  The problem is that the cube is as high as you are.  You cannot see the top face of the cube unless you jump.  In order to get on top of the cube, you're going to have to run toward the cube as fast as possible, jump up as high as possible while slamming your body into the side of the cube, and try to get your hands on top of the cube.  Then you will need to try and grip the top face of the cube with your hands and swing your lower body up to the top of the cube until you can get one leg on top, and then sort of roll the rest of the body over.  Damn.  I'm sweating just thinking about it.

Now imagine that the top of the cube is slippery, and you have no ability to grip the top of the cube.  You can run and jump and slam your body into the cube as much as you want.  Once you get your hands on top of the cube, they're going to slip right off.  If your hands slip off the top of the cube, then nothing else matters.  No amount of planned technique for how you're going to swing your body or what you're going to do with your legs will matter.

In my experience, what prisms have allowed me to do with vision therapy is get a grip.  Prisms have allowed me to grab and hold onto the top of the cube.  And now I can begin to think about what I am going to do next.  The stepping stool analogy isn't bad, but this analogy, although more verbose, is more accurate and precise.

I can speculate why using the prisms work in this way, but I don't frankly know.  I've used them in this way in the past and not with the same results.  But now, for whatever reason, prisms are working for me, and very importantly they make sense to me.  For the longest time, I always had this expectation that stereopsis was suddenly going to pop in for me as Sue describes her steering wheel suddenly popping out at her.  What does that mean?  Does that mean the eyes suddenly go from strabismic to straight?  Part of me, and I'm embarrassed to say it, thought that that's how it worked.  Not really, but kind of.

The prisms seem to free up energy to enable you to build on other skills like fusion and motor ability.  Without the prisms, it's just so much work to get superimposition, nevermind fusion, and the effort takes so much energy, nothing is left for building these important skills.  By freeing up energy, you're enabled to get a grip and build the important skills, and then ramp up a smooth gradient of progress.