Black Bars in 16:9 res with 16:9 monitor, how to replicate?

[Fedex15]

Hi, I want to add black bars in a 16: 9 resolution on a 16: 9 monitor. As seen in the image I have been able to do it, but I do not understand how it works. My monitor is an LG e2350, GPU: Amd Raden r9 270x, CPU: Ryzen 5 3600. What you try to do is stretch a 16: 9 beef, but not that it is centered, but that it stretches to the sides but keeping the horizontal stripes (up and down). I have achieved it by overclocking the monitor at 76Hz, changing the value of Total from vertical to 1195 lines, with that my monitor reaches 76Hz, without changing that value I could only reach 74Hz, but what I try to know is how those values work, such as you can replicate being black bars in 1600x900 60hz for ex. Thank you