Normally, KVC looks up for a property key and acts only after the property key is found. The other approach is about acting on an object's property in the lookup process. There is a lookup method in Objective-C core and its keys are used as selectors.
The following line of code is how you implement this lookup method:
objc_msgSend(myObject, mySetterSelector, myValue);
Note
This method is very similar to the manual implementation of the instance variable's setter, but instead of using the key to form a selector to do a look up, it uses the selector itself as the key.