The Knives in the Dark node in Smoke Bomb has a 70% chance (with 2 points) to apply a stack of Shadow Daggers every time Smoke Bomb applies slow, which by default is once per second, and due to requiring one point in the Swirling Fog node with 25% increased frequency should be every .8 seconds. With 4 points in Swirling Fog it should be every .5 seconds.
However, when testing the rate seemed much lower, so I tested Smoke Bomb 35 times with 1 point in Swirling Fog and 35 times with 4 points in Swirling Fog (no other nodes besides 2 points in Knives in the Dark) and the average result per cast was half of what it should be. With 1 point it was 1.7 Shadow Dagger stacks per cast, and with 4 points it was 2.6, whereas it should be 3.5 and 5.6.