All 3-technique tackles are not alike. Defensive coaches continually search for ways to make their defensive linemen more effective. One of those ways, which was later adapted to the Tampa-2 defense by Tony Dungy and Monte Kiffin, was to slide the defensive tackles away from the strength of the offensive formation instead of playing them in even alignments over the offensive guards. This "undershifted" front makes it very difficult for the offensive line to double team the 3-technique, or in this case, undertackle.
Here's how the differences look in the playbook and on the field:
With the strong side (TE side in this diagram) guard essentially uncovered, the defensive line has shifted away, or undershifted, from the strength of the offensive line. The strong side defensive tackle plays over the shoulder of the center and the weak side end plays a loose 5-technique outside the tackle, leaving the weak side defensive tackle (our 3-technique/undertackle) isolated against a guard. In many ways, on passing downs, you've schemed yourself a third defensive end.
Sapp wasn't the first player to ride the undertackle position to NFL fame and fortune. Before the birth of the Tampa-2, the Minnesota Vikings (under Floyd Peters and Kiffin) paired defensive end Chris Doleman and undertackle Keith Millard in a stunting under front defense. In 1989, Millard set a record for sacks by interior defensive linemen (18) that still stands today.
The lineage of great undertackle includes many of the league's other most successful pass rushing defensive tackles. John Randle, the first undertackle in what would become the Tampa-2 defense, racked up nine consecutive seasons of ten or more sacks. La'Roi Glover's 17 sack season in 2000 came as an undertackle. Kevin Williams, Rod Coleman, Vonnie Holliday, Tommie Harris? All have had very successful seasons playing 3-technique on defenses frequently using underfronts during the last five years. The Giants nickel pass rush that used four defensive ends to wreak havoc on offensive lines early in 2007 frequently moved Osi Umenyiora or Justin Tuck into undertackle-like roles.