Hi,
Yes, it is true that if you cover the intake, and there are no air leaks, then it will stall the engine as the engine is drawing in air as part of the combustion process. I guess this only really works if you have a large hole in the system as the engine will not be able to run off the small amount of air being sucked in through a join crack etc.
Diff gaurds.....well it depends whether you going rock crawling of gentle greenlaning and what type of gaurd you were thinking of.
Steve