> And if you are buying an instrument where you can lose more than you invested, the approach maybe wrong? :-)
This is precisely why shorting can lose more than you "invest", because you're not buying an instrument, you're selling it with the intent (or promise, depending on what kind of instrument it is) to buy it back later, hopefully at a lower price.
There are, as said, depending on juristic regime, products which do not let you lose more than you invested.
On top of this comes national regulation: E.g. in some EU countries, retail traders are exempt from s.c. "margin calls" and the broker is required by regulation to "just close and not ask for more"
I'm not familiar with turbos, but to me it sounds like a CFD? How does a short position on a turbo prevent you from losing more than what you "put in"[0]? If you're shorting underlying X at price Y with a turbo, and price moves to Y+10, you're going to lose 10 times the leverage factor. You could have stop orders, but those are not guaranteed to fill at a price that would cap your loss to a desired amount.
> the broker is required by regulation to "just close and not ask for more"
Some American brokers will also forcibly close your position instead of issuing a margin call. Do you mean that under those national regulations, the broker is required to eat the losses?
[0]: in quotes, since with a short position it's not really the case that you put something in.