Fotonoticias, Getty Images

The one man who knows the answer, Jen's longtime hairdresser Chris McMillan, spilled to why he helped the actress chop her long locks into a short and sassy bob.

"I'm loving bobs again. I love the girl in the YSL ads. It's a sexy look. The reason that I'm loving a bob at the moment... There are so many long, layered hairstyles and-don't get me wrong, I love that. But The Real Housewives of Beverly Hills all have that. You know what I mean? And what's the opposite direction you can go in?" McMillan said.

"It's nice to take the extensions out of the hair and do something more natural. It's gotten out of hand. But I can't stop watching it."

McMillan calls Jen's new do a hairline bob, because it follows the jaw down.

"It's shortened in back and longer in front. It's angled and inspired by Vidal Sassoon haircuts and Julie Christie from 'Shampoo'," he told Allure.

"It was just time for a change. We had a blast doing it. And we've always liked this haircut. It's about keeping things simple. The way Celine and Calvin Klein and Jil Sander and all these clothing lines are keeping it really simple and chic. I really feel that fashion right now. All the styles are back to when I first started doing hair. And I love it."

If you want the cut (and we know thousands of women will) what should you ask for at the salon?

"In the back it angles up-it was cut at the nape. I love it when it's slightly angled. The angle adds a modern edge to it. To keep it modern, it's not too blunt but also not too layered," McMillan says. "It kind of has a choppier edge. The ends are cut into, but mainly they're all one length."

In other celebrity haircut news, see what Justin Bieber's hairstylist had to say about his newly cropped 'do.