You guys know Deriv ?
https://eu.deriv.com/trade-types/multiplier/

They have a "Multiplier" type of contract, a bit like binary options, but they claim that you cannot lose more than your bet, but you can win more ?
I don't understand

If it works the way they announce it, it has an inherent edge to it. Just always call put and you'll be a millionnaire

so

what is the catch ?

Jeff