I was watching this video on YouTube, which showed several slow-motion shots of bullets impacting Prince Rupert's drops. One thing I noticed is that there's a noticeable delay between when the bullet strikes the drop, and when the drop recoils backwards.
It's most noticeable in the first shot, which starts at about 1m48s and lasts until about 1m55s.
I took a few screen shots to illustrate my point. These were taken at equalish (done by hand) intervals. I added the purple line. Note that the rightmost edge of the drop stays roughly flush to the line for frames 1-5, but then moves a few pixels in frame 6. It's even more striking in the video: the drop seems to hold almost still, and then suddenly lurch backwards.
One thing I did notice: the tip of the drop breaks off (without shattering the whole drop, as explained at 2m16s) just before the drop moves backwards. I'm guessing it has something to do with a shock wave traveling through the drop, reaching the tip, and then... something? If that's the case, then why does the drop move back as soon as the tip breaks? I would expect it to be further delayed by the time it takes for the shock wave to move back up.
What exactly is going on here?