Most fatal accidents in cars involve side impacts.  The human brain
reacts very sensitive to sideways motion, and the active head restrain
system will essentially deploy an air bag next to your head along the
side of the window to cushion your head.  It is a good investment, one
of those you hope you'll never need.

Dirk

I looked at cars at the dealer today.
The active head restraint is similar to what's on the Volvo.  The head rest
moves forward in a rear collision to reduce head movement that could cause
whiplash.
The salesman advised against getting the rear airbags if children will be in the
back seat, as they're more likely to be injured and to lean against the door.

Rick DeBay