Machine guarding is all about preventing dangers
Guarding Contact: Machine guards have to prevent human contact with hazards, being them poking hazards, cutting tools, sharp edges, blades, dented wheels, you name it - if it is harmful, and it's the guard's job to keep you safe from it. Of course this doesn't mean that it has to be fool proof - proper operator training is important too.
It shouldn't be easy to bypass: A MG shouldn't be easy to remove or bypass, you don't want people to inadvertently remove or disable a safeguard mechanism without noticing only to get their hands on the machine's blades. Therefore, proper MG procedures are taken to keep guards in place and hard to remove.
Preventing Accidents: Accidents happen, and they happen a lot. Let's suppose you are working with a machine that has moving components like dented wheels and an object falls from your hands into the machine - it would be transformed in a deadly projectile due to the pressure and hurt you when it comes out shooting - that's why proper machine guarded machines will prevent falling objects and other external accidents to transform the machine into a danger.
A guard doesn't interfere: Let's face it, humans in general are lazy. As such, you can't have a machine guard that hinders the way people work or they will just go ahead and remove or disable such guard, because his life would be apparently much easier.
A guard shouldn't be a hazard.
This is pretty much a given, guards shouldn't be hazards themselves, they shouldn't be able to puncture, crush, cut or slash through stuff.
Be easily maintained.
A guard should be easily maintained in order to make sure it is maintained frequently. Machine guarding has proper maintenance as one of its core approaches, as well as proper training and perfect guarding.
So, bottom line, these are some simple prevention measures that can, when applied, make sure that machine guarding does its job to prevent accidents and to allow operators to handle machines in a safe and productive way.
Remember, machine guarding is all about prevention.