Well it's not that hard to understand really if you understand how withdrawal limits work. The basic idea is that when a transaction comes from an address which has a withdrawal limit preventing the address from being quickly emptied, then a merchant can have much more confidence in any transactions received from that address since the merchant knows how many blocks they have before the customers address can be emptied out. As long as the merchant can get the transaction they received from the customer into a block before that amount of time, the merchant is guaranteed to get paid.

The one issue with the idea is that an attacker may be able to prevent the merchant from getting their transaction into a block by flooding the network with high-fee transactions which may get priority status from miners. The general approach for solving this problem is to make sure that older transactions always get priority over new transactions, although I'm unsure exactly how well it's implemented in Cryptonite and how well it will work in practice because I haven't done a whole lot of testing with withdrawal limits. Another approach recently suggested involves contracts between merchants and pools, but I forget the details.