Meaning of "shoots straight to your mouth and unbuckles your heart"?

Hello everyone. I am reading a novel, and I came across this expression. Could you please let me know its meaning?

Perhaps it was time for me to leave as well, though I didn’t want to yet. And suddenly it came to me. I should call her, shouldn’t I? Just call her. And say what? I’d figure something out. Time I did something—always waiting for others to do—tell the truth for a change, engage, for crying out loud. I don’t want to be alone tonight. There! She’d know what to say to that. She’d keep the conversation going; and even if she had to say no, it would be a kind no, as in: Can’t, lying low, you see? Ah, but to hear her say it that way, Can’t, lying low, you see? like a reluctant caress that starts but then lingers on your face and shoots straight to your mouth and unbuckles your heart. I reached for my cell. She was the last person to call—hours ago. We’d exchanged numbers while still waiting on line, and she said, Let me call you instead, this way you’ll have my number too.

- André Aciman, Eight White Nights, Second Night

This is a novel published in the United States of America in 2010. This novel is narrated by the nameless male protagonist who meets Clara at a Christmas party in Manhattan. Now the protagonist is thinking how he should act now by calling Clara and saying what he feels.

In this part, I wonder what the underlined expression means.

What would it mean to "shoot straight to one's mouth" and "unbuckle one's heart"...? o_O Thank you very much for your help.