They are for the most part based on futures contracts of the VIX. They are very deceptive in many ways and one definitely can't "ASSUME" anything with them. Just because the SPX or broad market sells off doesn't mean they will rally strongly or visa versa. And with futures on VIX being in Contango which means the ETN which is based on 'nearby' futures, that contract expires and the next month out becomes the front month that the ETN is now tracking which is a different price on each rollover to the next contract. Add to the issue is many of the VIX based ETNs are double such as TVIX and that alone is a nightmare to chart with the way double ETFS and ETNs are calculated. Bottom line is these ETNs will 'DETERIATE IN VALUE' over time even in a sideways market. If you overlay any of them on top of a VIX chart you can see this clearly. The only time they rally strongly is when fear grips the market regarding a major selloff or percieved major selloff. Then the put buying of options which the VIX is based upon increases rapidly and a higher premium is paid for this put protection for everyone's stock portfolio. That is what I understand it to be all about. Very easy to lose your shirt trading VIX based ETNs even with a pullback in the market. I know I posted a chart of it some time ago and it would have been a great trade but in afterthought I wished I hadn't because of the danger of financial loss with them. It's like having a pet rattlesnake. It will bit you in the ass 1st chance it gets. And if you get long which is tempting now with the test of support,rally and mild higher pullback recently unless this market really comes down hard with a lot of fear of the future this ETN will languish and deteriorate once again to new lows. And looking at the high beta of TVIX you can lose big on it even with a pullback in the markets. I know looking at the price at $110 on the Oct low looks mouth watering "IF" there is a strong pullback in this market with the present price around $17 but just know what you are risking before getting into this trade. But it is tempting for sure,lol.......................