For archer (also some casters), it's just that you successfully triggered a skill trait's effect aka a proc while classes like tank and melee have similar stuff but they are more of an actual combo as they are obtained through successful skill execution in a pattern. For you case I think you are referring to the Heavier Shot trait, whenever you use Heavy Shot skill you will have a chance that your Straight Shot will flash the dotted lines that you mentioned, if you use Straight Shot when it's flashing ( which it's called Straighter Shot) it will always deal critical damage. You can check out the skill and traits description under Additional Effect to see which triggers what for effective combat.

As for Chaining, you can only obtain combat chaining by defeating monsters of your level and above, regardless of the order or type of monsters you defeated (Like defeating Monster B after two chains of monster A won't break the chain). Anything below that will not get you chain succession, this also won't break you chain.

Long explanation but I hope it helps.