This is pretty fantastic:
https://www.reddit.com/r/ffxiv/comments/45wsu4/huge_blm_cast_speed_exploit_discovered_someone/
The comments are kind of cesspit, but someone dug up a video of the method that was being used:
https://www.youtube.com/watch?v=SpEiNB_vpmo
tl;dr: BLM abuses the stutter-step cast mechanic by some means to give himself ridiculously short cast times. Posts logs on fflogs anyway.
(Personal übermensch highlight from the comments:
"It's not an exploit. It's not a bug. It's not a hack. It's an oversight--and it's been in the game since Heavensward launched.
It's been publicly mentioned in-game and on Reddit before, and no, this BLM isn't the first or the only. This practice is widespread in top-end statics, especially Japanese ones, and so it should come as no surprise that they don't want you scrutinizing their logs and accusing them of exploiting.
One of the problems with the FF14 community is that you're all followers. You all follow one strategy for every boss fight, you all use one parser, you all copy the same DPS "rotations"--even when those strategies, parsers, and rotations are wrong or inefficient. Knowing that 99% of the FF14 playerbase just copies what's already been done, it shouldn't come as a surprise that certain mechanics are never taken advantage of by the majority of players.")