Throwing Knives
Throwing Knives has a very simple condition. When you Crit with another item, use this. This makes Throwing Knives perfect for quickly dealing damage. The drawback of Ammo items is, as it sounds, Ammo. If you run out, you can't deal anymore damage and will likely lose the fight. (Unless the Sandstorm finishes the job)
Throwing Knives is a powerful aggro tool, but when combined with other items, it can turn into a "one-shot" build.
Repeater
The Repeater is the other Ammo item that creates a loop with Throwing Knives. "When you use another Ammo item, use this." The word "use" is an important distinction as Pistol Sword, a similar item, won't create a loop.
The goal of this build is to get Repeater to 100% Crit Chance. This will then loop Repeater and Throwing Knives back and forth. Knife will trigger Repeater, which will trigger Knife, which will trigger Repeater, and so on until you run out of Ammo.
This all happens within a fraction of a second, making this build effectively a "one-shot" build.
Flashy Reload
Important Patch: Flashy reload now states "When you Crit with an item, reload another item +1 Ammo"
The last piece of the puzzle is Flashy Reload. If you have 100% Crit Chance on both Throwing Knives and Repeater, this Skill "should" turn the two into an infinite loop. I say should because I have run into long fights where Knives ran out of Ammo.
I can't say for certain that this combo isn't infinite, but from personal experience there might be some interaction underneath that messes up the combo.
Regardless, having this skill makes the combo near infinite, meaning you can technically have a Bronze Throwing Knife and still kill opponent's with enough time.
Lastly, the build can still work without Flashy Reload. You will just need good damage and Ammo on your Throwing Knife to kill enemies with one go.
Crit Items
Crow's Nest
Crow's Nest is often the go to Crit item for this build as it affects your entire board. It will go from 25/50/75/100% Crit Chance, making it a great way to get 100% Crit on both your Knife and Repeater.
Swash Buckle
Swash Buckle is another common Crit item due to the bonus Damage. This lets you convert Crit Chance beyond 100% into extra damage for your Knife and Repeater.
Combining Swash Buckle with Crow's Nest also gets you to 100% easier without needing to fully upgrade Crow's Nest.
Note: Swash Buckle is a Gold item minimum, meaning you won't find it too early in the game.