When you defeat an enemy, +200 Initiative.
Killed an enemy.
If used in conjunction with very fast weapons (like the Knife) or weapons that can kill many troopers in a single attack (Launchers or the Grenade Benie), it can give the trooper enough initiative to attack the enemy again before the boost runs out, making him tear through enemies unimpeded until he runs out of ammo or is stopped by a counter-attack.
Spies benefit the most out of any other Trooper. With the right weapon & skills — for example the Semi-auto shotgun, Loader, and Battle-Ready, Spies will go on a killing spree and can potentially win a battle singlehandedly.