I'm currently reading the paper dealing with the mathematics in NXT ("A Probabilistic Analysis of the Nxt Forging Algorithm") and the author describes an attack I don't understand. He states:

"the attacker spends in the main chain and waits for some confirmations, and then broadcasts a (long) sidechain that would be accepted by the nodes as the main one". And continues with:

"Therefore, in this section we analyse how many blocks in a row can be typically generated by a given account over a long period of time."

I really don't understand what is happening in this kind of attack. I thought there were no classic sidechains in NXT? And why is it necessary to create several blocks in a row?