From the poem "Phenomenal Woman" by Maya Angelou, please explain the stanza beginning with, "Now you understand / Just why my head's not bowed."

The final stanza in Maya Angelou's poem "Phenomenal Woman" wraps up the poem's assertions about the speaker's status as a woman who exudes power and attracts men without artifice.
When Angelou states, "Now you understand / Just why my head's not bowed," she is asserting that she is proud of herself, her body, and her qualities. She doesn't have to resort to drastic means to get attention, and does not "shout," "jump about," or "talk real loud" in order to attract attention to herself. She has such a sense of self-worth that she is able to recognize that it is a privilege for others to see her pass by them. In this way, she defies the stereotypes attached to femininity and rejects the opinions of women who meet the beauty standards of the time and men who may usually expect those standards in the ladies they pursue.
Rather, Angelou continues to assert the natural characteristics that contribute to her extraordinary nature—the sound that her heels make when she walks, the texture of her hair, the appearance of her hands, and so on. These are qualities which no one can replicate and which occur innately to her. The stanza closes with the repetition of:

'Cause I'm a woman
Phenomenally.
Phenomenal woman,
That's me.

This is Angelou's battle cry of pride, confidence, and security in her identity. It contains both amazement at her status as a woman and delight in how spectacular she is. This is ultimately the message of the poem—a call for all women to embrace themselves exactly as they are, regardless of the observations and opinions of the outside world.