In 1984, when Winston said to Julia, "You're only a rebel from the waist downwards," what did he mean?
Julia rebels against the physical privations that come with being an outer Party member. Inner Party members enjoy all sorts of privileges denied to the rank and file, such as good coffee, wine and nice apartments. Julia rebels against the puritanical hypocrisy of the system by having sex with men who can offer her the comforts she wants, including sex, but also consumer goods, such as make up. Unlike Winston, she is not concerned with the nature of truth or the nature of reality. She doesn't really care if the Party says two plus two equals five or whether or not the Party invented the airplane. She simply wants to have some pleasure in her life (and part of this includes the sense of victory that comes with beating the system) so that she can experience something beyond an endless round of toil, cheap gin, and rage.
While Winston's statement sounds like a put down, in fact, he admires her pragmatism, and it is important to remember that without her know-how, she and Winston would never have gotten together. She approaches him and arranges the first meeting. She gets them luxuries like real coffee.
In the end, she and Winston fall in love, and she embraces his idea of joining the Brotherhood and rebelling in a more overtly political way against the Party.
As the previous answer states, this is a reference to the essentially physical nature of Julia's rebellion. Unlike Winston, whose rebellion is ideological and intellectual, Julia defies the Party chiefly because it denies her the physical pleasures that she so craves. The most obvious way in which she does this is by engaging in the act of sex. She tells Winston she's had several lovers in the past - and appears distinctly unmoved by the fate of at least one of them, a Party member who committed suicide before he could be eliminated by the Party. "A good thing too," is her response, "otherwise they'd have had my name out of him when he confessed." She appears distinctly cold here, although she does show genuine warmth towards Winston.
Julia also wants other physical indulgences, like make-up and chocolate. She wants to blossom out into a woman in the way that the Party denies; therefore when she takes off the shapeless sash of the Junior Anti-Sex League to which she ironically belongs, this is highly symbolic. She really wants the finer and sweeter things in life, which the Party is so dead set against. Her physical needs and desires orchestrate her rebellion.
Winston is insinuating that Julia is only capable of rebelling with her body, not her heart or mind. Winston enjoys the rebellion of her body because it is in the act of sex that she primarily rebels. However, as Winston tries to encourage Julia to think about Goldstein's book or to think about the possibility of joining the Brotherhood, her only interest in it is to satisfy him. Ideologically, she is not old enough to remember what the world was like before the hostile dictatorship of the alleged Big Brother.
Winston becomes more human throughout the majority of Book 2. This is due, in large part, to Julia. He is experiencing a relationship that he failed to ever have before. This comment comes as a result of Winston longing for the next piece of human intimacy: companionship. Winston longs for someone who will think similarly and be capable of discussion about the conditions of the world they live in.
Julia has been a rebel with her body with scores of men. She goes to meetings with the Anti-Sex League just to throw off the Thought Police.