This event occurs when one of the Ambient properties change (type Ambient. and in the dropdown box you see all the properties).
For instance, drop a usercontrol on a form, and change the form's font property, the ambientchanged event will occur.

I'm not sure how to use this, I thought it applies to an ambient property of the direct parent (container) of the control, but when I put the testcontrol on a picturebox and I change the backcolour of a form, this event is also raised.