Ok this is really hard to explain but I'll try.
First of all: Hypothetical DPS, you do 200k DPS on boss and 400k DPS on leg.
Basically, the faster we kill the legs single target the more
POTENTIAL DPS loss we have.
Lets say he regens his leg every 60 seconds, over a 7 minute fight thats 7 legs. 1 leg has 6mio HP, 7x6= 42mio DMG. It doesn't matter how much DPS you have on the legs, in the end of the day you'r maximum damage on legs will ALWAYS be 42mio.
Ok so he spawns it every 60 seconds, you stand in the circle and do 200% normal dmg. You do 400k DPS and kill it in 15 seconds, you then go on the boss and DPS him for 45seconds with 200k DPS which is 9mio DMG. All together you did 15mio DMG to boss in these 60 seconds. Now you do that 6 more times. 6x15mio=90mio DMG
Now instead of killing the leg in the circle you kill it outside the circle. It takes you 30 seconds instead of 15 seconds, but you still do 6mio DMG. And DPS the boss for 30 seconds which is 60mio DMG. Alltogether 12mio and 6 times more 12x6=72mio DMG This isn't really that important it's just more important that you understand this.
But now take into consideration a cleaver. Lets say rogue with blade flurry. He does a little less DPS on single target with that enabled. Let's say 50% less DPS (actually exacetly 50% since his energy regens 50% slower). Instead of 200k he only does 100k if he would single target with flurry. But now there's also a leg there that takes 200% dmg more. He is DPSing boss for 100k and leg for 200k, together 300k. Now instead of taking 15 seconds to kill the leg it takes him exacetly 30 seconds. But WAIT! He did 6mio DMG to the leg and a nother 100k DPS to the body. That is 100x60=6mio DMG. 6+6=12mio. And he still has 30 seconds SPARE before the next leg spawns! So he turns off flurry and has 200k DPS on boss again 30x200k=6mio DPS so 6+6+6=16mio do this 6 more times = 96 MIO DMG!!! That's 6mio DMG MORE! If you were only to DPS legs and then body!
Ofc the numbers aren't 100% correct but the math is here just to show you that a cleaver would do much more DPS if he wouldn't focus the leg directly.
It doesn't matter if we're behind on the leg a little we can always swap some single target DPS to it and we won't loose any DPS through the whole raid BUT WE WILL GAIN DPS IF HE GET'S TO CLEAVE AS LONG AS POSSIBLE BEFORE HE REGENS THE LEG FROM BEFORE AND WE ACTUALLY HAVE TO KILL IT!
More dumbed down:
5 people DPS the leg, we do exacetly 100k DPS on leg. Which means we kill it in 1 minute flat. 100k / 5 = 20k per person. Now 1 rogue in there is cleaving instead of single target. He does half (10k) the damage on single target or 20k on two targets but actually does 30k DPS since the leg takes double damage. 10k on boss 20k on leg. that means 5*20k DPS on leg which is still 100k and an aditional 10k DPS on boss! BOOOOM!
Even if it does only 35% dmg on single target. 35% of 20k DPS = 7k DPS, he still does 14k DPS on leg and 7 on boss which is 21k alltogether, which is still more then 20k DPS, the leg in this case would die a little slower but we kill it way faster in reality anyways, not a problem really. Basically if your cleave hits for AT LEAST 35% of your normal attack YOU USE CLEAVE! And let's not even go to the lenghts of passive CLEAVE like ghoul from Unholy DK. That's just insane here.
Same goes for Warlocks and Shadow Priests, or any DOT class. Dot up the leg and DPS body. Just make sure your dots kill the leg before next spawns and you stand in the circle. The DPS increase on legs isn't there to BOOST our raid DPS directly. It's there so we can kill the leg just with small attacks and it doesn't take our full attention in other words. Casters with DOTS, check on dummy if JUST your dot's do at least 35% of your damage.