- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks how many seconds are left until your next main-hand swing, off-hand swing when Off Hand is ticked, or Auto Shot or wand shot when Ranged is ticked, compared with the number of seconds you set. While you are casting or channeling, a melee swing uses your full swing time.
Why you might use it
Compares the seconds until your next main-hand or off-hand swing, or with Ranged ticked until your next Auto Shot or wand shot (before the first shot of a fight it reads the full weapon interval), for rotations that are timed around the swing timer: a Classic Warrior casts Slam right after a swing so it does not delay the next one, or queues Heroic Strike only near the end of the swing. With no off-hand weapon, an off-hand check is false (and true under NOT). It does not exist on Retail or WoW Forever, where addons cannot read the combat log it relies on. See also Weapon Swing Time.
Example
Your main hand weapon swing time remaining is greater than 1.5 seconds
Settings
What each setting in the condition editor means, from the game's own help.
Off Hand - Should this condition affect your main or off-hand weapon. If this is checked, it will affect your off-hand weapon, otherwise it will affect your main-hand (or two-handed) weapon.
With no off-hand weapon equipped, an off-hand check is false (and true under NOT).
Ranged - Measure your Auto Shot or wand shot instead of a melee weapon. Choose Off Hand or Ranged, not both. The time remaining needs the combat log, so it works on Classic Era, Burning Crusade and Mists of Pandaria but not on Retail or Forever.
Operator - How to compare Swing Time Remaining with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - The amount of time (in seconds) until your next weapon swing.