9-Trick is an exploit in Codename Eagle.
Due to the high amount of damage over time, the time the grenades spend release Necro Gas, and the fuses of the projectiles, this exploit allows very effective spawn camping and exit/entry blocking. Due to the nature of the projectiles being able to bounce off of the ground and walls, you could cause a large amount of damage and block off lots of territory, without the enemy getting a line of sight. However, due to vehicles and fast player movement speed, the Gas Weapon is next to useless in direct combat and therefore isn't used much and isn't a very big threat, and isn't very game breaking.
Tapping the corresponding number key will equip whatever weapon is in that slot - for example, tapping 2 will make you draw your revolver. Whenever a weapon completes it's deploying animation it is ready to fire - regardless of when the weapon was last fired. This means it is faster to tap the 9 key and draw the Gas Weapon and then fire, than to fire the Gas Weapon and wait for the long recoil animation and cooldown.