How I think Rubber Ducking works for me:

Explaining things to others - imaginary or not - engages the parts of my brain imagining how they will receive and understand it.

This is primarily so I can phrase it in a way other people can understand. But that imagined model of other minds is often smart enough to imagine what they would answer.

Which I guess means my "social brain" is smarter than my "thinking" brain in some ways.

Don't know how universal that is. My brain tends to be an outlier.