I thought about this and watched the video again, and I've changed my mind just a bit. It looks like, instead of being accelerated, the whip end is experiencin

I thought about this and watched the video again, and I've changed my mind just a bit. It looks like, instead of being accelerated, the whip end is experiencing reduced drag in the vacuum between the two shock waves. We can see that it travels right behind the first shockwave, not lagging at all, and this is exactly what you would expect if it is both the source of the shockwave and the beneficiary of the vacuum behind it.