the Changes Pending checkbox cannot be set by the user. The checkbox is set when the corresponding Item is used in a Change Process.
These two Methods do the job:
With "Where Used" you can see, that these Methods are used in ItemType Server Events. But beware - that´s only half the truth. Most of the time, these Methods are triggered by LifeCycle transistions.
These two Methdos trigger the checkbox update quite often:
When you create a custom change process, you must extend these two with your own functions.