QUESTION:
How is the inflation of the airbags triggered?

ANSWER from Tommaso P Rivellini on December 19, 1996:
The events that occur just before first contact with the Martian surface
include airbag retainer release, airbag inflation, solid rocket motor ignition
and finally the cutting of the Kevlar bridle that connects the lander with the
backshell, parachute and rockets. All of these events are scheduled seconds in
advance by software executing on the lander's on-board computer. The software
figures out when to do these things based on monitoring digital altitude data
that comes from a radar altimeter also mounted on the lander. This altimeter
will produce a continuous stream of altitude measurements starting from
approximately 1.6 km above the surface until the lander is about 300 m above 
the ground. By that time the software already has a good idea of how high 
it is and how fast it is falling under the chute (roughly 62 m/s). That 
is also about the altitude that the airbags retainers are released and 
inflated. The rockets are fired when the lander is about 70 m above 
the ground. The timing of the rocket ignition and bridle cut are 
calculated so that the lander (inside its protective airbag cocoon) will 
come to a stop at about 12 m above the ground.