Incase some are confused, A/R is the port just before the turbine wheel. Some manufacturers use other terms to describe this area. Holset and MHI refer to it as cm^2. This ports purpose is to vary the way the incoming exhaust attacks the turbine wheel causing a desired reaction.
A loose description of its purpose would be a garden hose and those little plastic yard flowers that spun in the wind.
If you took that open hose without a nozzle (your A/R) and let the water hit the flower (turbine wheel), it would make it spin. Just not vary fast. During this period the pressure within the hose (back pressure) will be very low. This also will allow the largest amount of water to leave the hose.
Take the same situation, but place your thumb over most of the opening, and you will see the water leave the hose at a higher velocity and the flower accelerate (spool) much faster than previous.
You have just changed the a/r or cm^2 to a lower value.
BUT, what you will also notice, is a substantial increase in the hose line pressure (backpressure)
Over time (RPM), this restriction hurts / chokes your engine and kills its efficiency (HP).
Enjoy!

