This is the schtick to intercept melee
All gunmen know that their best shot is in the few miliseconds when the enemy is close at hand, but before he actually comes into contact. Endless hours of drill and training have honed your reflexes to the degree where you instinctively shoot at anything coming right at you without even thinking.
Whenever someone tries to move close by you or moves up to engage you in close combat, you get to take a shot at them out of sequence, after they move but before they get to act. This reduces your shot count by one.
This only applies if you are armed, ready (Fast Draw helps) and not engaged. Once an attacker manages to get into contact with you, you must move away to a new position to use this advantage again.