I have high spirit regeneration and can therefore fire lots of Bells out, I think that is why I noticed this effect. But I believe this will be very usefull for Nirvana users especially and other builds that can dish out lots of bells at once or over a span of time.
Here it goes:
Do not aim WoL point blank at whatever monster/mob it is you are targeting. Instead when encountering a mob let it surround you and point away from the mass of them, not at them like everyone always does. WoL is an AOE spell, it completely surrounds the monk, even his back and sides (if you put on Blinding Light you will actually see very clearly the many circular blue auras that seem to echo or vibrate on the ground around you and on monsters, the more monsters the more echoes). So what I have noticed is that by not directing WoL in a full frontal assault on mobs there seems to be almost a % chance that only some monsters will be knocked back and the rest will simply not. I can practically prove it because Blinding Light stuns monsters when I get a critical hit, yet many of the monsters are not being knocked back at all and are instead sitting still being killed with critical hits which can clearly be seen as they are stunned and have stars above their heads. This effect happens when the blast of light is angled correctly so that the echoes or vibrations hit the monsters and not the actual blast/spear of light itself which seems to be doing most of the knockback and I'm guessing almost all Monks focus them directly at monsters instead of positioning themselves to use the sides and back side of your monk when directing WoL.
Now this is not to say that monsters will not be knocked back at all at these certain angles but I believe it highly has to do with the direction of your WoL and the position of a monster/s as well as the proximity of the "echoes" to the monsters themselves. i.e. The edges of the echoes/vibrations seem to not knockback nearly as much but still provide critical hits. And I'm talking about a much lower % chance of knock back when done correctly (thinking at least 50% based on observation alone). I believe this is why WoL is so effective on large mobs in FoM. Think of yourself as Moses parting the Red Sea. Everything directly in front of you is cleared away, yet the water (monsters) on the sides and behind are still there, some are being knocked back but not nearly as much as the monsters taking the spear of the WoL fully on.
Here's another way of looking at it. Think of a clock. When attacking a group do not hit them with anything in between 10 o'clock and 2 o'clock fo your monks body. Everything else will have a better effect in terms of knockback and still cause the same amount of damage. If you don't believe me go test it out on your own. I think this will help a lot with group play as well as solo and maybe now CoB WD's and CM wiz's won't have to complain as much!