Is there a possibility that it's because they're taking on way more risk than most of the volatility selling funds you've seen? I mean these things are 90-95% probability bets - much like the CHF peg was.
The 90-95% probability definitely helped. And there should be something else about how to close the position or hedge it. I do not believe they are delta-hedged using SP futures during the whole time of their positions since the Sharpe ratio could not be that high. Meanwhile their opportunistic hegding/roll over/buy out were very good, otherwise the flash crash or summer 2011 would show much bigger drawdowns.
Speaking of other funds like Chicago based LJM or Tampa based ITB, they behave much like slightly deleveraged version of passively short S&P VIX future indexes with re-balance. To get their performances you can simply use VIX futures with greater liquidity and less transaction costs.
Being said that, I am only guessing what they were doing from their "advertising" documents and my limited simple backtests.
