The NFL tweaks its rules every year, but a rule change before the 2005 season garnered particular attention. In an effort to prevent injuries, the league banned the so-called horse-collar tackle (a maneuver that involves dragging an opponent down from the back of his shoulder pads), which had caused a few high-profile injuries.

The rule change was big news, especially in Dallas, where some called it the “Roy Williams Rule.” Williams, a Cowboys safety, employed the technique several times, most notoriously in Philadelphia on December 19, 2004, when he brought down Eagles wide receiver Terrell Owens by the collar. Owens jerked back violently as his cleat caught the turf. He suffered a broken leg on the play and missed the rest of the regular season plus two playoff games. Williams also injured Tennessee Titans receiver Tyrone Calico and Baltimore Ravens running back Musa Smith with horse-collar tackles.

After all the off-season publicity, though, a funny thing happened during the 2005 season: The rule change had next to no impact. NFL officials only called the penalty once – and that was against an offensive player. On Monday night, October 3, Packers quarterback Brett Favre threw a short pass to tight end David Martin. Carolina Panthers cornerback Ken Lucas stepped in front of the pass, intercepted it, and ran it back 32 yards before Martin caught him from behind and dragged him down, drawing the first and only penalty for a horse-collar tackle of the 2005 season.

Many fans, players, coaches, and television announcers have criticized league officials for failing to enforce the ban on the horse-collar tackle, but those criticisms are based on a lack of understanding of exactly what a horse-collar tackle entails. There’s more to it than just dragging a player down from behind.

To commit the penalty, the tackler must grab the equipment on the back of the ballcarrier’s neck and pull him straight down. Grabbing the jersey, the top of the shoulder pads, or anything else does not constitute a horse-collar tackle. And although the league has changed many rules to protect quarterbacks, it’s still legal to haul down a quarterback that way while he’s in the pocket. The horse-collar tackle ban also doesn’t protect running backs when they’re near the line of scrimmage and between the tackles.

Giants running back Tiki Barber complained last month that the officials weren’t upholding the horse-collar tackle ban after Eagles defensive end Jevon Kearse wasn’t flagged when he pulled Barber down from behind. But the distinction was that Kearse yanked Barber down using only Barber’s jersey, and the rule states that to draw a flag, the tackler must grab the player’s equipment. It seems like a strange distinction, but the NFL wanted to outlaw the most dangerous tackles while still doing everything it could to ensure defensive players weren’t hindered from catching a ball carrier from behind.

Why didn’t the penalty make a difference this year? Did players adjust their technique when making tackles, or did the officials turn a blind eye when it happened? The answer is most likely neither. The real reason the horse-collar tackle wasn’t a big issue in 2005 is that it wasn’t really a big problem in 2004. If Owens, one of the league’s most visible players, hadn’t been hurt on a horse-collar tackle, few people would have given the matter much thought. In fact, throughout the 2004 season, most people had never even heard the phrase “horse-collar tackle.”

A computer database search of newspaper and magazine articles shows that the phrase “horse-collar tackle” appeared in only one article in 2004,an account in the Baton Rouge Advocate of a game between Alcorn State University and Southern University. But in May of 2005, it suddenly became the talk of the NFL off-season, when the league voted to ban it. Of the league’s 32 teams, 27 voted in favor of the ban. Williams’s Cowboys were one of the five teams to vote against the change, with team owner Jerry Jones saying the new rule was ambiguous.

The NFL said its officials wouldn’t throw flags on ambiguous plays, and the lack of flags this year has proven the league right. The horse-collar ban was a wise move by the NFL because it protects players in an already harsh sport, but it wasn’t the major change many expected.

Mr. Smith is a regular writer for FootballOutsiders.com.