Twitch's basic attacks apply a stack of Deadly Venom, dealing 2 / 3 / 4 / 5 / 6 true damage per second for 6 seconds. This effect can stack up to 6 times for a minimum of 12 / 18 / 24 / 30 / 36 and maximum of 72 / 108 / 144 / 180 / 216 total true damage over the duration.
Rat-Ta-Tat-Tat has no cast time and does not interrupt Twitch's previous orders.
Twitch's attacks during Rat-Ta-Tat-Tat can outrange turrets (850 range opposed to the turrets' 800) meaning he can damage them during Rat-Ta-Tat-Tat without taking damage from them.
Twitch's attacks will pass through structures and terrain, but will not damage structures unless they were directly targeted for the attack. The exception is the Nexus - Twitch's attacks will not deal any damage to it. This is likely a bug.
For purposes of Runaan's Hurricane, attacking with Rat-Ta-Tat-Tat counts as a single auto-attack against a single targeted enemy, and as such will only launch one pair of ordinary single-target bolts to nearby enemies. However, the range bonus applies for all three bolts.