In reply to GrahamD:
> Stick another krab between your plate and your main belay krab. Adds significantly more friction to the system.
**Whoa**
There's a very important distinction between two very similar setups that have completely opposite effects. Simply following the shorthand above on skinny ropes couold be very dangerous.
An extra krab, through the rope only, between the plate and the main krab, will **reduce** friction. It's a well-used way of making thick/sticky ropes go through grabby plates.
An extra krab, through the rope *and the belay loop* (duplicating the belay krab), *and of the same size*, will increase friction.
I'm sure you meant the latter, but you described the former.
The caveats in the latter are important - and those, plus the ease with which you can make a dangerous mistake, are the reasons I'd never advocate additional krab for increasing friction (the other way round - trying to reduce friction but failing - is failsafe).