There's a correct way to flex the club up. The secret is all in the (right) wrist. Our Top 100 Teacher Jon Tattersall explains.
This is your standard weekend-play hinge (below). Yes, the club is up, but only because you've hinged your left wrist toward your left forearm. It's a one-way hinge, so even though you've created a nice 90-degree angle between the shaft and your forearms, the clubface is open. It looks good, but shots hit with this type of backswing are likely to miss short and right.
In addition to hinging your left wrist up (below), you need to bend your right wrist back and rotate your forearms, so the face is at the same angle as your left arm. Now the club is on plane and loaded for a powerful strike. See you on the green, player!