Christopher Duan Johnson (born September 23, 1985) is an American football running back who is currently a free agent. Born in Orlando, Florida, Johnson emerged as a senior for East Carolina University where he broke out for 2,960 all-purpose yards and 24 touchdowns. He was drafted by the Tennessee Titans in the first round of the 2008 NFL Draft, after running a then-record breaking 4.24 seconds in the 40-yard dash at the NFL Combine. In 2009, he won the NFL rushing title with 2,006 yards (the sixth of only seven players ever to gain over 2,000 in a season), and broke Marshall Faulk's record of total yards from scrimmage with 2,509.

Johnson had his professional debut with the Titans on September 7, 2008 where he played in the season opener against the Jacksonville Jaguars, which the Titans won 17-10. Johnson rushed for 93 yards on 15 carries and had 3 receptions for 34 yards and a touchdown. In only his second career game, Johnson ran for 109 yards on 19 carries, including a 51-yard run, and 2 receptions for 12 yards against the Cincinnati Bengals.  On September 28, Johnson rushed for the first touchdown of his career against the Minnesota Vikings. Johnson finished the game with 75 all-purpose yards (61 rushing, 14 receiving) and 2 touchdowns. In only his sixth professional game, he rushed for 168 yards and a touchdown on only 18 carries against the Kansas City Chiefs on October 19, 2008. Johnson finished the season with 1,228 yards on 251 carries for a 4.9 ypc and 43 receptions for 260 yards and 10 total touchdowns in 15 games. He led all rookies in rushing yards per game with 81.9 ypg.  Johnson finished second in AP Rookie of the Year voting to Matt Ryan and was elected to the 2009 Pro Bowl.  Johnson split carries with running back LenDale White for the duration of the 2008 season. White finished the 2008 season with 773 yards on 200 carries for a 3.9 ypc and 15 rushing touchdowns. They nicknamed this RB tandem "Smash and Dash." Smash referred to White due to his power, and Dash referred to Johnson due to his unusual quickness and speed. Entering into the 2009 season, however, Johnson dismissed the nickname of "Smash and Dash" and named himself "Every Coach's Dream."

Using a quote from the above article, answer the following question: What did Chris Johnson specialize in?
Dash referred to Johnson due to his unusual quickness and speed.