yeah i know that, but for example in our pot, we had chelsea etc. What if Barca would have got chelsea from their pot. We still have chelsea in our pot right? And the balls are unmarked so techincally someone shouldnt be able to remove the chelsea one...
No man, all the second placed teams were put in one pot. And when one second placed team is drawn, Chelsea for example, there isn't another ball with Chelsea's name on it.
This is what happens so you could better understand:
Pot1: Chelsea, Sporting, Arsenal, Vilarreal, Atletico, Inter, Real, Lyon
---------
Pot 2: empty pot
Pot3: Juventus, Juventus, Juventus, Juventus, Juventus, Juventus
Pot4: Barcelona, Barcelona, Barcelona, Barcelona, Barcelona, Barcelona, Barcelona
Pot5: Liverpool, Liverpool, Liverpool, Liverpool, Liverpool
Pot6: ManU, ManU, ManU, ManU, Manu, ManU
Pot7: Bayern, Bayern, Bayern, Bayern, Bayern, Bayern, Bayern
Pot8: Porto, Porto, Porto, Porto, Porto, Porto
Pot9: Panathinaicos, Panathinaicos, Panathinaicos, Panathinaicos, Panathinaicos, Panathinaicos, Panathinaicos
Pot10: Roma, Roma, Roma, Roma, Roma, Roma.
Now, when a team from the first pot is drawn, for example Chelsea, they know that Chelsea can't get Roma, ManU and Liverpool so they pick one ball from all the remaining pots and put it in the empty pot.
Then they take out the remaining balls from the empty pot and they draw another team from the first pot. Depending on who can that team play, they again pick up balls from the other pots and put it in the empty pot.