Restraining the human head in racing crashes -- thereby eliminating the overwhelming common cause of death in the sport in recent years -- isn't a simple proposition.

Tethering race drivers' heads safely and practically is a scientifically sophisticated process.

Several devices have been invented during the past 20 years, but most have proved ineffective.

Safety-equipment manufacturer Bill Simpson last year came up with a device similar in appearance to the HANS -- for "head and neck support."

But independent testing by Dr. John Melvin, in conjunction with Wayne State University and General Motors, showed Simpson's device only "slightly reduces the neck load," Melvin says.

That means that in high-energy crashes, "you'll still have the same outcome" -- unsatisfactorily violent head movement.

Melvin, out of scientific curiosity, ran experiments using tethering straps that ran vertically -- parallel to the neck -- and anchored to the shoulder harness rather than a collar. That didn't work either.

"We got enormous neck compressions," Melvin says, "enough to make you a quadriplegic."

The alternative system "just stuffed the rest of the body into the helmet" in crash-dummy testing.

Here's how the HANS device works:

It is made of ultra-strong composite materials, such as ballistic Kevlar.

A collar is held to the driver's body by the safety-harness system already in use in all organized motor racing.

Two, sometimes three, tethering straps extend from the collar to the driver's helmet.

The straps allow head movement, up and down, side to side, in normal racing conditions.

Under crash conditions, the straps and collar keep head movement uniform with that of the torso, which is allowed some brief movement before being restrained by the harness system.

By keeping head and body movement uniform, the HANS prevents the extremely violent movement of the head independent of the torso.

Preclusion of this violent head movement prevents extreme G-loading of the head and neck. Such forces in the past have led to broken necks, severe brain-stem stretching and basal skull fractures, which are the most common cause of racing fatalities.

The HANS has been undergoing scientific testing, mainly using crash dummies on "sleds," for more than 11 years at the facilities of General Motors in Warren, Mich.; Mercedes-Benz near Stuttgart, Germany; and at Wayne State University in Detroit.

These tests have shown definitively that the HANS does precisely what its inventor-developer, Dr. Robert Hubbard of Michigan State, claims: It prevents head and neck injuries with no adverse side effects.