Pinning is the idea that ‘they’ will attempt to close out a session or expiration at a particular spot price, presumably a high volume option strike so that neither Put nor Call buyers get paid. While this possibility exists for lighter volume underlyings, as liquidity increases, the viability for this to happen gets reduced greatly.
For vol ETPs, this is even less logical. As [demonstrated], it is the percent changes in VX contracts that determine the value of the benchmarks that all of these ETPs use to determine prices. If ‘they’ want to pin UVXY, they will most likely not be able to pin SVXY, VXX and others at the same time. They would also be trying to move a much larger market (VX futures) to effect a much smaller one, which is the opposite of how an effective leveraged manipulation would work.
So the argument here is that ‘they’ are going to drive the prices in the much larger VX contract market, so to make UVXY finish at a strike price to save a few ticks on a few thousand contracts. The risks and costs to do this make little sense but traders love to imagine nefarious reasons behind price movement.