It is not down to 6 people forever not colluding. It's down to 6 people not having colluded at one point in time in the past, each having been scrutinized during the procedure, with post-hoc inspection of the software and hardware. If those 6 people decided at any point after the procedure that they wanted to collude, then it would be too late for them to do so.
Essentially they each produced a private key, and if each of them revealed their private key to the same party then that party could derive a master private key that would allow them to (among other things) mint Zcash for free (privacy wouldn't be broken). Assuming that any one of the 6 did in fact destroy/corrupt his private key without revealing it, then the collusion opportunity is forever lost.
Essentially they each produced a private key, and if each of them revealed their private key to the same party then that party could derive a master private key that would allow them to (among other things) mint Zcash for free (privacy wouldn't be broken). Assuming that any one of the 6 did in fact destroy/corrupt his private key without revealing it, then the collusion opportunity is forever lost.