> "Enter open mode"?  If so, what would the "/pattern/" do?  
 
It scrolls the screen up one line, and opens the line it found.

 > (Is there some easy to get to place that documents how open mode acts/works?)

The easiest way is to just log in to any random slowlaris, sunos, hpux, irix,
aix or osf1 box and try it.  Maybe an old ultrix...I suppose even sco would do.

	% ex /etc/motd
	:1o

It doesn't really need separate documentation: just imagine a 1-line vi
window and you've got it. The only difference is that it advances to
the next line on the screen when you move down or search. I use it on
the above-named systems when I want to edit something and still see what
was on my screen.  On NetBSD one goes with vi-as-ex or ed for that purpose.