Your baseline line foot should be back in order to force the ball-handler toward the baseline. As you approach the ball-handler, snap your shoulders and head back to help slow yourself down.
Basketball is played on the fingertips and the balls of your feet. Everything you do involves correct footwork. Some players are naturally quicker than others. But a player's effective quickness can be greatly enhanced if he/she uses proper footwork.
A naturally quick player who lacks good footwork skills can be beaten (or contained) by a player with sound footwork fundamentals. Watch the Villanova men's team play... they have probably the best footwork of any team in the nation.