In 1984 by George Orwell, why does Julia encourage Winston to volunteer for munition work on one evening a week?

In Part II,
Chapter 3 of ,is a much better rebel than .  She knows that "If you
kept the small rules, you could break the big ones."  She knows that--by volunteering for
the Junior Anti-Sex league or decorating for Hate Week or volunteering Winston at the munitions
depot-- she and Winston will not be profiled and surveilled as much by the Though Police,
telescreens, or Big Brother.  As such, they can later coordinate a bigger rebellion, maybe
involving the Proles.

She [Julia] even induced Winston to
mortgage yet another of his evenings by enrolling himself for the part-time munition work which
was done voluntarily by zealous Party members. So, one evening every week, Winston spent four
hours of paralysing boredom, screwing together small bits of metal which were probably parts of
bomb fuses, in a draughty, ill-lit workshop where the knocking of hammers mingled drearily with
the music of the telescreens.

This reflects the theme of
"appearance versus reality."  Julia looks like a party-member, but instead she is a
"rebel below the waist."  She shouts and yells at Goldstein's image during the Two
Minutes Hate, but really she'd rather yell at the Party members.  In a society where everyone is
watching everyone else, one has to look like a flower, but be the serpent under it.

Ironically, all of Julia's volunteering may have been for naught, as Big Brother
suspected her (and them) all along.  Or, it may have been part of her plan as a double-agent:
she may have been a spy all along.  We or Winston will never know.