I think mostly due to the 2 second rule, it is very very simple to pull off disease twisting. The only delay I've seen is occasionally waiting a fraction of a second for a disease to tick before reapplying, but that only happens with very high haste (glove enchant) and/or bloodlust. In my experience, the hardest thing about it is getting out of the groove of applying one disease first every time.

It is a different story if you do it without empower rune weapon, as you will end up waiting on runes and possibly losing DPS.

I think you miss how the trick works.

You don't refresh your diseases at 21 seconds.

You refresh your longest remaining disease at 18.1 seconds (2.9 seconds remaining), and then your other disease just ran out as you refresh it.
Ebon Plaguebringer never drops, and Rage of Riverdare only drops once every 40 seconds, and that's assuming there is no other unholy DK in the group.

There are 2 ways to do it:
1. Use ERW at the start of the fight, this is particulary effective if you pop a gargoyle in your first rotation, which most do. Simply use:
IT>PS>SS>BS>BS >Garg >ERW
SS>DC>SS>SS>DC
PS>IT>SS>BS>BS>etc
(note due to ERW your BS come after your UF runes, thats the reason i put SS before the BS's; the dps loss is neglectable over the course of a fight anyway)

2. Use another set of runes that refresh at the same time. That means you reapply your diseases in your first rotation.

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I've done this successfully for entire fights, because once you get used to it it's really simple. ICC has enough fight where you never break combat (Saurfang / Festergut), so you can use the ERW methode. On the fights where you do break you can swap to methode 2.

The gain is pretty significant too. Because with this methode you eliminate the extended downtime in diseases you normally get.

Normally you get a tack at 18 seconds. Since you refresh both diseases at 20 seconds, the next tick happens at 23 seconds. That is 5 seconds without disease ticks.
This methode eliminates that 2 seconds of down time on both diseases totally (or reduce it to 0.1-0.3 seconds extra), meaning you get nearly an extra disease tick per rotation for both diseases.
Instead of 6 ticks you get 7 ticks. Thats a 16.67% increase in disease damage (and WP damage ?), Not really sure how much diseases account for but that's an easy 1-2% dps increase for nearly no effort.
Yes you might lose RoR on your blood plague application every 40 seconds, but that loss doesn't compare to the gain.

Potlol's post still gives a good indication of how the system works;
http://elitistjerks.com/f72/t72364-u...7/#post1544510

It doesnt scale with crit, just ap and haste.

Wouldnt GoBS raise the value of Subversion quite significantly? Especially if were doing a Reaping-less build.

Edit: Also, does the Icy Talons haste affect our ghoul/gargoyle/aotd?