An exhaust cut out sends the exhaust out of a pipe before it reaches the muffler.
Normally the exhaust is flowing through the muffler which quiets the vehicle down.
But an exhaust cut out is an opening between the muffler and headers and when opened it sends the exhaust out through that under the vehicle instead of through the muffler.
Exhaust cutouts are an opening in the exhaust system between the headers and mufflers.
An exhaust cutout does help performance and increase horsepower although it results in a nosy vehicle and can get you a ticket in some places.
Clearing out the exhaust more quickly does impart more horsepower.
But, don't forget, bypassing the muffler makes for a noisy vehicle.
Every motor vehicle with an internal combustion engine shall be equipped with an exhaust muffler system in good working order and in constant operation to prevent excessive or unusual noise, and no person may use a muffler cutout, bypass, or similar device on a motor vehicle on a highway.
If you are currently running a stock system, you actually only need one cut-out installed before the muffler.
The life span is quality dependent, read your reviews and get a good one.
I have seen them last 6 months and over 2 years personally.