Since Phase 2 started, I've decided to dabble in the other two races, Protoss being the final one that I've yet to pick up.

Now that I'm starting, I always wondered how exactly the Colossi's attack works. I decided to investigate the matter and thought I'd share my findings with Team Liquid.

This might be super basic to those of you who have been playing Protoss for a while - I honestly have no idea. I'm hoping this will be vaguely helpful to someone out there. If anything, you get to see some pretty pictures of lasers.

The Test

Take a big group of marines and move them towards the Colossi, then have the Colossi fire on the moving group. I've repeated the test moving the marines in random directions after the beam fired and with range. Both support the findings below.

Step 1: Damage Calculation


The Colossi's attack animation begins and damage is calculated. The units which will receive damage are chosen at this time.

Step 2: Damage Dealing (Phase 1)


The attack deals damage first to the units on the outside, but this leading damage only proceeds the rest of the damage by a few frames.

Step 3: Damage Dealing (Phase 2)


The rest of the units are fully damaged as the beams sweep together.

Some interesting things to note:
Damage isn't calculated by what the animation touches, it's calculated the moment the attack animation starts. The animation itself is just for show. If you notice in Step 3, the beams are clearly in the middle of the rest of the marines, but only that one line of marines has received damage.

Also, damage is done exactly perpendicular to the direction the colossi is facing and extends in a very narrow line. This means that to maximize damage, you should position your colossi such that they're not flanking the opponent's concave, but instead standing behind your row of units (presumably tanking). Of course, maximizing damage isn't always what your priority is - sometimes you'd rather position your colossi in safer locations.

Source: http://www.teamliquid.net/forum/viewmessage.php?topic_id=136711