As you approach the line on a beat, you may be moving 6 knots in the direction you’re headed but only 4.5 knots directly towards the line.  4.5 knots is called your Velocity Made Good (VMG).  The idea of shooting the line is to head up into the wind when you’re close to the line.  Your VMG will then immediately jump to your actual speed (e.g. you’ll be doing 6 knots directly towards the line), and it will then begin to decrease as the wind slows you down.  How quickly you slow down depends on your momentum (heavier boats take longer to slow down) and the wind strength.  You want your bow to cross the line as the moment your actual speed drops to your previous VMG (e.g. when you’ve slowed from 6 knots to 4.5 knots).  This way you haven’t sacrificed any speed towards the line and have in fact crossed it more quickly.  In this photo, the third place boat may be able to steal second if he shoots the line and the other two don’t.  Of course, exactly when to head up is very difficult to judge, so it would be a good idea to practice when the race isn’t close.  That way, when you need the skill, it will be in your arsenal.  It’s also generally a better idea to do this at one of the ends of the line since it’s much easier to judge where the line is there