The video shows someone installing a gps bike tracker, however there is not much room in the top of the head tube, so he hammers down what's already in there in order to make room for the tracker.
In this video, (starting at 1 minute 30 seconds)
I wanted to make a custom gps tracker for my bike as I'm into electronics, but I have no idea what this 'hammering' does. What is this part he's altering? And is it safe to do so?