The Phantom Hit occurs a static delay after the basic attack was fired (independently of melee or ranged), and will occur even if the attack was destroyed (i.e. Wind Wall) or popped (i.e. untargetability).
The stacking and Phantom Hit itself occur on-attack and do not interact with abilities that act like a basic attack (i.e. ability granted Headshot).
One-use on-hit effects, such as Siphoning Strike and Sheen, remain one-use, as the on-hit effect will be consumed by the initial strike and be unavailable for the second.
It is important to know the difference between an on-hit effect (which Phantom Hit will apply) and an on-attack effect (which Phantom Hit will not), these effects are listed below.
Phantom Hit does not cause you to attack twice. Effects that trigger when you attack or whose counters increase when you attack will only apply once per attack, regardless of Phantom Hit. This affects abilities such as:
For effects such as Vi'sDenting Blows, the game will pro-actively use the champion's empowered attack animation if the attack would trigger the effect due to Phantom Hit - e.g. Vi will perform her power animation on every 2nd attack (with Phantom Hit).
Guinsoo's Rageblade's unique combination of traits make it something of a niche item. It is best on basic attack-based champions who benefit from both attack damage and ability power and rely on on-hit effects for a major portion of their damage (Jax, Master Yi and Kayle). For such a champion, however, it is recommended to build the item as soon as possible, as the power spike it creates is usually enormous.
The Phantom hit will proc both Titanic Hydra and Ravenous Hydra essentially doubling the area of effect damage, however the Titanic Hydra is superior in this aspect as it also deals on-hit damage on the primary target.
Unique Passive - Wrath: Deal 2.25 bonus physical damage on-hit for every 1% critical strike chance, up to 225 bonus physical damage on-hit at 100% critical strike chance, but can no longer critically strike. Damage applies to structures.
Unique Passive - Seething Strike: Every third basic attack triggers a Phantom Hit on-attack that applies on-hit effects to the target unit after a 0.15 second delay.
Unique Passive: Basic attacks grant 8% bonus attack speed on-attack, after a 0.15 second delay, for 5 seconds, stacking up to 6 times for a maximum of 48% bonus attack speed. Melee champions can fully stack it with their fourth basic attack. While at 6 stacks gain Guinsoo's Rage. Guinsoo's Rage: Every third basic attack triggers a Phantom Hit on-attack that applies on-hit effects to the target unit after a 0.15 second delay.
New Passive: Basic attacks deal 5 (+ 10% bonus AD) physical and 5 (+ 10% AP) magic damage on-hit.
New Unique Passive: Basic attacks grant 8% attack speed, 4% bonus AD, and 4% AP for 5 seconds, stacking up to 6 times, granting a maximum of 48% attack speed, 24% bonus AD, and 24% AP. At 6 stacks, gain Guinsoo's Rage. While at half stacks, melee champions' next basic attack fully stack Rageblade.
Removed Unique Passive: Basic attacks grant 8% attack speed, 3 attack damage and 4 ability power for 5 seconds, stacking up to 6 times, granting a total of 48% attack speed, 18 attack damage and 24 ability power. At 6 stacks, gain Guinsoo's Rage.