Why raid chain timing is used, and what it means

What is raid timing

To start with "What does it mean when someone lists times with a raid setup" Often times you’ll see something like “Eleanor 0.9/Mk99 1.5/Gabrielle 1.0/Tinia 1.5"

Which means "You'll wait until the chain status timer (under the boss's health) reaches this value to activate that chain skill. So "Eleanor 0.9/Mk99 1.5" means "when the chain status timer hits 0.9, press the button for Eleanor's chain skill, then wait for the chain status timer to reach 1.5 and hit mk99’s skill etc."

Most of the time raid runs are tuned with the expectation that your lead will have a maxed out weapon skill regen speed and you'll be using an item that gives you +10% WSRS (Mino neck or gold pocket watch when maxed) sometimes there'll be exceptions to that which should be noted, and sometimes you'll need to delay your weapon skill for a specific thing which should also be noted. (Videos of course you just have to watch for this) but most of the time the expectation is that you will be stunning a boss with your weapon skill as soon as possible

Rarely an accessory without weapon skill regen is used instead, and even more rarely a mirror accessory with 12% weapon skill regen is used. Note that the weapon skill regen speeds add up, so if your weapon is 2% short of max WSRS and you have a 12% mirror accessory, you'll have the same timing as someone with max weapon WSRS and a mino neck (at the cost of doing less damage)

Why do we care about timing in raid

Now... "Why do we do this" what's the point of using specific chain timings...

Bosses have to complete actions before they move on to the next action, for example Viper won't stop in the middle of his multi-punch animation to use a different move, he has to finish his move before he can switch to the next one, the exception to this is if you interrupt his move with a weapon skill stun or chain skill, Now if you hit a chain skill above 0.7 seconds or so, they just stay stunned, so they can't start their next action, if it's below that point, they can start their next action.

Bosses select their next action based on some criteria, it might be randomly selected (typically from 2 options), it might be a set pattern (Sometimes it's "do X then do Y or Z randomly"), it might also trigger off the position of the heros or the timer. The point is the boss decides based on its rules and then starts its next action and does the entire animation for that action unless interrupted. If they are interrupted they move on to the next action when they stop being stunned (whether it's the sequence in a repeating pattern, or some action determined randomly or by specific conditions) They do not finish the animation they were currently in the middle of.

Boss-specific considerations

To use a timer example, Viper has a special condition where if the global timer is below 35 seconds his next action will be the teleport bomb thing. Since that move is the one that causes the most problems for your team, your objective is to prevent him from acting during your chain until the global timer is below 35, then let the chain skill timer get below the 0.7 point, so he starts the teleport-bomb thing and have your chain skill still land to interrupt the animation.

Another factor here is that chain skills have their own animation time, the stun happens when the chain skill lands not when it starts so if you want your chain skill to land at 0.6 seconds left on the chain timer it has to start some amount of time before that.

Now "Teleport and start throwing spirit bombs" is all one action, it takes maybe 0.2 seconds between the start of the action and the actual teleport, so you do have a very small window in which you can hit him with any old chain skill and prevent the teleport after it's started, but that's a pretty narrow window. Chain skills like Shapira's, Tinia's and Gabrielle's skill will "follow" the target (they'll hit even if the target teleports) so you can actually interrupt Viper's bomb throwing even if he does teleport away, as long as you start your chain skill before the teleport and land it after the 0.7 mark.
So when figuring out what chain timings to use you need to figure out "What moves are problems" and "How do I prevent or interrupt those" or in the case of a boss with a pattern it's often "How do I get them to skip the problematic part of the pattern and spend most of their time in the portion of the pattern which causes the least issues" (Fairy and Director are good examples of this)