What does PUSHJ do? (was Re: was "C on the PDP-8 ... ")

[Sean 'Captain Napalm' Conner]

It was thus said that the Great Brad Parker once stated:
> 
> heh.  I remember puzzling at the the "pushj"'s and "popj"'s in the FOCAL
> source code.  This was before I had seen a pdp-11 or dec-10.

  What does PUSHJ actually do?  I remember reading about it in Steven Levy's
Hackers, but never got what was so special about it.

  -spc (Saves the register J, right?)