Dear Chaim,

This is discussed in Shulchan Aruch 645:7 and Mishna Brurah there: "If the separated sides look like two (leaves) it is pasul".

It is very difficult to describe the Psul of Hemnik. Basically, Rishonim describe the break to be like a "fork" in its width (i.e. that the leaves are pointing in different directions). And if it is separated in that way for even a Mashehu (minute) length it may be Pasul. (This is besides the Psul of Nechleka, where the middle leaf is split a certain length even if not noticeable.)

The accepted opinion is that the Psul of Hemnik is only on the first day (although there is an opinion which forbids for all 7 days).

All the best

Reuven Weiner