How does Lady Macbeth's will break down her husband's?

"My dearest partner of greatness" is what Macbeth calls his wife in the letter he writes her to tell her of the witches' prophecies. Yet as your question quite correctly implies, it's famously Lady Macbeth who wears the trousers in that particular relationship. You can probably find good quotes and examples for this across much of the early part of the play (and certainly make some interesting comments about when Lady M's influence fails to persuade her husband - during, for example, the famous Banquo/banquet scene).

I'll focus on in one scene to show you her in action. Macbeth has decided that they are not going to kill Duncan - his good angel has won the argument, and his conscience has prevailed.

MACBETH
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.

LADY MACBETH:
Was the hope drunk
Wherein you dress'd yourself? Hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love.

He says they're not doing it, and she goes for him full-on. Did you dress yourself in a sort of drunken mood, she asks? Why are you so cowardly, so 'green' and 'pale'? If you're a coward in this instance, she says, your love must, like your bravery, be pale and wimpy and not good enough.

She goes straight for his masculinity to break down his will and over-ride it with hers. "You're NOT a man", she's saying, 'UNLESS you do what I say".

MACBETH:
Prithee, peace!
I dare do all that may become a man;
Who dares do more is none.

LADY MACBETH:
What beast was't then
That made you break this enterprise to me?
When you durst do it, then you were a man...

It's all there in that last line: when you dared to do it, then you were a man. Unless you do what I want, you're not a man. And she even mentions what (we presume) must be a child that the couple have lost:

I have given suck, and know
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this.

He breaks. And he consents: "if we should fail?". He's not got as much resistance as she's got determination. Her will wins the day.

Hope it helps!