So on NA cars what's the deal with them creating more torque on the lowend when they have a restrictive exhaust?

I understand they will make a lot more power up top if you get rid of the exhaust but then you will lose low end power. I have been hearing a lot about the lack of backpressure will make you lose low end torque, but how does this really happen?

What's the real working of this loss of power? I'm looking for one of those insightful lengthy responses!