Hey, I have two cents!
"The body should be triangular, the mind circular. The triangle represents the generation of energy and is the most stable physical posture. The circle symbolizes serenity and perfection, the source of unlimited techniques. The square stands for solidity, the basis of applied control."
I can't quite remember where I obtained this quote but it makes sense to me!
This is a quote that has been around for a while, and I think it's a good place to start.
For me the Square, Circle, and Triangle flow into each other - Nage starts within the solidity of the Square, aware of the eight directions that are possible for attack - then moves into a Triangle entry and posture for proper position and angle, and then finally allows the Circle - or actually (Spiraling) Sphere - to manifest, which is the basis for proper execution of all Aikido Technique, or in my mind, the Release.