My loader has the hydraulic self leveling feature. I have a little lever below the FEL joystick that actuates the feature. Some loaders use a mechanical system for self leveling.
If the feature is engaged, then it will maintain the angle of the FEL attachment (bucket, spear, forks, etc.) to the ground as you raise and lower the FEL. You can still use dump and curl to get the angle where you want it and then the self leveling feature will maintain that angle.
Self leveling makes it easier not to drop or spill materials as you raise and lower the FEL. You just pull back or push forward on the joystick to raise or lower the load. Without the feature you need to use curl and dump as you lower and raise the FEL to keep from spilling or dropping materials. Dump and curl functions still work while in self leveling mode.
I like the self leveling loader feature and use it almost all of the time.