In The TeXBook it says
The value of
\prevdepthat the time of the
\halignis used at the beginning of the internal vertical list, and the final value of
\prevdepthis carried to the enclosing vertical list when the alignment is completed, so that the interline glue is calculated properly at the beginning and end of the alignment.
In section 15.2 of TeX by Topic, it says
At the beginning of a vertical list
\prevdepthis set to
-1000pt, except in an
\noaligncode contained therein, where it is carried over from the surrounding list.
I'm able to understand the sentence in The TeXBook, but could not understand the sentence in TeX by Topic. Could someone give some examples about