Premium / discount to net asset vale (NAV) is another factor in price changes..
HVU closed today at $6.52 but Horizons calculates NAV at $6.57 ( http://www.horizonsetfs.com/pub/en/etfs/?etf=HVU&tab=overview ). So if all factors impacting HVU were to remain statc overnight then HVU would open at $6.57. The Horizons learning library gives an overview of this at http://www.hbpetfs.com/pdf/ClosingVsNav.pdf.
Today's difference of NAV to closing price could be explained by mild pessimism re: overnight events possibly impacting negatively on HVU 's opening price tomorrow, or more likely just a minor tracking error.
I'm pretty sure that Horizons does not calculate or post the NAV in real-time, just at daily close. You could calculate your own approximate in-day NAV by looking at the change of the two underlying VIX futures contracts and the current ratio of the contracts for that day. A premium or discount might indicate a selling or buying opportunity. I would think this effect would typically be pretty minor and subject to rapid change, but it's something to keep in mind.
As an aside, does anyone know when Horizons does the daily partial roll into the further contract. Is it a set time every day, throughout the day, or random ? Is there a real-time source for HVU NAV? Does HVU typically trade at premium or discount? Thanks.