As far as I know, there are only two things that can set off the air bag:
1) A collision with an object* at a speed of 35 km/h or higher (measured by speedo and accelerometers)
2) An unlucky person stabbing away at the steering column with a screwdriver trying to remove the airbag
As for how the structure of the vehicle is meant to be for proper deployment I'm not sure. A certain degree of crumple is expected in the front, accommodated by the bullbar if present (hence the need to test them).
It really is best to leave that sort of thing to people qualified to deal with it. I'm not, so I don't play with it - although I have mounted a winch to it.
* The fact that this object is larger than a housefly is left out as it's just bloody common sense. However, I have heard of airbag deployment from hitting a camel, so the limit is somewhere in between.