Im very hard smiter, so I have experience in this =) You can queue spells wnen they are on CD if that CD is lower then your queue setting. 500ms for example.
This is the same latency bug seen with casting Renew right after a single-target heal with a cast time as Holy in single-target Chakra. The upshot for that, as a smiting Disc. priest, is that the CD reduction on Penance isn't very useful on the last Smite before a Penance - by the time the CD reduction registers, Penance is usually off or nearly off CD anyway.
My sense is that the spell queuing system does not allow you to queue up a Penance when it's on CD, but I could be wrong about that. If you can, it makes the latency bug much less of an issue. Can anyone speak with certainty about this?
Thats how it works (and that bug really imroves your dps):
Start cast 1st Penance, Penance CD starts, 2 sec cast time -> queue 1st smite
Start cast 1st Smite, 1.5 sec cast time -> queue 2nd smite
Start cast 2nd Smite, 1.5sec -> after small latency Penance CD lowered by 0.5 from 1st Smite -> Penance CD remains 0.5sec, so queue 3rd smite
Start cast 3rd Smite, 1.5sec -> after small latency Penance CD lowered by 0.5 from 2nd Smite (useless) Penance off CD -> queue 2nd Penance
Start cast 2nd penance (6 sec CD starts) 2 sec cast time -> after small latency 2nd Penance CD lowered by 0.5 from 3rd Smite -> queue 4th Smite
Start cast 4th Smite, 1.5sec -> queue 5th smite
Start cast 5nd Smite, 1.5sec -> after small latency Penance CD lowered by 0.5 from 4th Smite -> Penance off CD so queue 3rd Penance
So first 6 sec of Penance CD was filled by:
2sec penance cast time, 3 sec of 2 smites and addition 0.5 effective cd reduction.
So we have 5 sec of pure cast within 5.5 penance CD. So we waste 2nd CD reduction and delay Penance by 1 sec.
2nd Penance has 5.5sec of base CD because of bug. So it was filled by:
2sec penance cast time, 3 sec for 2 smites and addition 0.5 effective cd reduction.
So cast time of 1 penance and 2 smites somehow (with bug help) perfectly fits within 5 sec.
You can cast without delay (Penance-Smite x2). And all Smites will reduce Penance CD. But one of them will reduce current penance CD and other will reduce future penance CD.
In world w/o bug we have to cast (Penance-Smite x3) or idle 0.5 sec after second smite.
But we dont need to. WoW engine loves Discs =)
Its a Smite-magic =)