She describes in a variety of areas how elite women's athletic performance catches up to men as soon as women gain access to similar quality training. Because men suffering from an insecurity in their masculine identity find physically powerful women threatening to this identity they strive to humiliate, threaten, or ignore any women trying to gain access to whatever aspect of strength is currently considered most "masculine".
A topic that I found personally applicable was when she addressed the feeling of being dissociated from ones body (with a corresponding feeling of depression). She argued that this stems for from a lack of experience using it, enjoying it, and feeling physically empowered.
One of the ways that this particular experience is hidden from women is through the fear of looking too masculine. If a woman strength trains, our culture, seems to silently scream that she'll bulk out and look like a man or at least one of those dread "lesbians".
I've had that particular fear for most of my life, which being trans, was perhaps a bit more realistic of a fear when I was going through a male puberty. Though now all these years after transition, I'm no more likely to look like a man than any of my other female relatives.
Additionally this possible link between depression and lack of exercise also seems intriguing. Considering many people in the US never actually use their bodies, perhaps our brains are wired to think of the lack of physical use as suggesting that we are useless, which then triggers a descent into depression.
Or alternatively since there are so few routes to feel a sense of power in our society for most people, being good at some physical task might give a very tangible sense of power.
The last part of the book focused on physical strength as a ward against fear of attack. The frail feminine woman is kept it her place through societies condoning of violent male aggression (through harassment and rape). She argues that by learning to be strong, competitive, and aggressive, women can look at an aggressor and say "I know I'm tough. If you attack I will fight back.", and frequently these bullies will retreat and look for easier prey.