Resolution Changer works by using a hidden Android API called IWindowManager. Originally Google restricted access to this API starting with Android Pie however they allowed developers to disable the API blacklist by tweaking a Secure Setting.

Resolution Changer first unlocks the blacklisted APIs and then applies the requested resolution and display density.

If you do not specify a height or width Resolution Changer will simply use the resolution that is currently set by the device.

If you do not specify a density Resolution Changer will try to calculate a suitable density based on the diagonal screen size of the device.

WARNING:
If you specify a broken resolution and are unable to fix it follow these steps:
1. Connect device to PC
2. Open CMD or Terminal (depending on OS)
3. Type these commands:
adb shell wm size reset
adb shell wm density reset