Real power, I’ve often said, is what you can do beyond your job description.

Google VP Marissa Mayer

The 180 women convening tonight at the annual “Fortune Most Powerful Women Evening With…” dinner in New York City know this — or they will before the night is done. Among the leaders with us will be Xerox XRX CEO Ursula Burns (who I will be interviewing at tonight’s event), Wal-Mart WMT Chief People Officer Susan Chambers, and Google GOOG VP Marissa Mayer, all three of whom happen to be “mentors” in a global program that Fortune runs In partnership with the U.S. State Department.

Also at the event will be New York’s most influential women — including Diane Sawyer, Barbara Walters, and Martha Stewart — but the real stars of the evening will likely be the 26 mentees who came to the U.S. from around the world to take part in this Fortune-State Department Mentoring program. Over the past month, we’ve paired each of these rising-star businesswomen, from 16 developing countries, with U.S.-based execs who attend Fortune’s annual Most Powerful Women Summit.

These mentees have spent the past couple of weeks shadowing the top women at Fortune 500 companies such as American Express AXP , Johnson & Johnson JNJ , and Dupont DD , as well as Google and Walmart. And yesterday when I met with these mentees, who have just finished their stints at the host companies, many told me that they’ve had “epiphanies” that will significantly alter their work back at home.

All for the good. This is power in the making.

The long-term goal, as the MPWomen mentors and we at Fortune view it, is to make a difference somewhere out in the world — yes, way outside anyone’s job description. As this year’s mentees — the sixth class since we launched the mentoring program in 2006 — prepare to return home, we’re urging them to “pay it forward.” Goldman Sachs GS , which has hosted a mentee from Nigeria this year, partners with Fortune on an annual award for mentee alums who most productively deploy their power back in their home countries.

If all this sounds very female, that’s because it is. As I write this Postcard, I’m sitting in a room at Goldman Sachs, listening to well-known entrepreneur Gerry Laybourne (who created the Oxygen cable TV network) speak to the 26 mentees. Laybourne is telling the group that women — even powerful women — tend to be lousy at self-promotion. “We don’t know how to toot our own horns,” she says, adding, “If you don’t toot your own horn, toot another woman’s horn.”

In other words, spread the power. That’s what tonight is all about.