http://www.cboe.com/micro/vix/vixfuturesprices.aspx will show the next two months VIX futures prices ( 15 min delayed) which HVU tracks. I suspect real-time data may be by subscription only. If anyone knows a free real-time source please post.
Remember that HVU is tracking those contracts and that VIX is derived from put buying activity in the options market. While this will generally trend inverse to the S&P 500 this will not always be the case. Recently you have seen S&P 500 and HVU both closing up on the day. This increase in put prices, and thus the VIX, reflects growing concern that the the current gains are unsustainable, that the Fed might soon begin winding down QE, that investors are looking for downside insurance for unrealized gains, or outright betting on a declining market (or some combo of all-of-the-above).
The Horizons ETF website shows HVU is currently tracking 12% May / 88% June which are month codes K & M on the above CBOE link. For a list of month codes, see http://cfe.cboe.com/tradecfe/Ticker_VIX.aspx. The ratio of which months HVU is tracking will shift through the month until the May contract expires, and the cycle starts over with June / July .
Currently, the May price is lower than the June price (contango) so the daily partial roll from May to June creates a hurdle to overcome before the ETF price can rise.The Horizons ETFs partially roll to the far month contract on a daily basis to spread out the effect of contango and avoid possible big swings on expiration day.
Horizons has a good learning library that is worth consulting, including http://www.horizonsetfs.com/Pdf/Education/MathOfHBP.pdf . The chart on page 4 shows pretty clearly the 'wasting effect' due to day-to-day price fluctuations inherent in leveraged ETF's. Read the articles, very informative.
Given all of the above, you wil see why leveraged ETF's are usually recommended as a short-term ( hours or days in my opinion ) rather than long-term or 'buy-and-hold'. It is critically important to understand this if you are investing in leveraged ETF's .
As far as gauging "possible wild swings" .... therein lies the beauty of investing. You will win or lose depending on your acuity and luck, so best of luck to all.
The above is my 'enthusiastic amateur' opinion, any correction or supplemental info is appreciated - thanks.