Question:
- What is Over triggering event? 


Answer:
- If you are triggering frame acquisition with an ExFSTrig signal and you attempt to acquire frames at too high a rate, some of the frame trigger signals that you apply will be received by the camera when it is not in a "waiting for frame start trigger" acquisition status. The camera will ignore any frame start trigger signals that it receives when it is not "waiting for frame start trigger". (This situation is commonly
referred to as "over triggering" the camera.

When the camera is over triggered, it will generate the Frame start over trigger event. It can’t be used as image grabbed event. 

Pylon is not further developing on the low level API. So I would suggest to use the high level API which come with Pylon events like “OnImageGrabbed” which is what you are looking for.