Here's a student many will tell you "lacks flexibility." He thinks it (sometimes, when I haven't seen him in awhile ), other instructors have told him he lacks flexibility, etc.
His hips sway right, his torso turns about 75°, and he lifts his arms up to "finish his backswing."
It's a bit better in the left photo here because he's been working on this for quite some time now, but even still you can see those trademark things: hips sway back, no secondary tilt, head rises, arms lift, turn isn't great.
On the right you can see him doing the wall drill. You set up near a wall. You note how much space you have between your trail hip and the wall, and then you put your arms across your chest and make a backswing while you strive to increase that distance. Make the gap between your trail hip and the wall get bigger. Voilà! Secondary Axis Tilt, hips going forward during the backswing (yes, a bit too much, but this is a drill, exercise, or "feel"), head not going up, more torso turn.
The proof, as they say, is in the pudding.
It's important to note that you don't necessarily actually want all of these to occur during your backswing. This drill reverses a lot of what you do (hips swaying instead of turning, moving too far right instead of staying "centered", reverse axis tilt). It's a great drill for an 18 handicapper who sways and wants to get better now.
As always, these are actual swings, not posed shots.
2017-09-15: Edited the title. Originally it was "Lack of Flexibility and the Wall Drill". We teach this to people who DON'T think they lack flexibility, too. Even kids.
2021-11-25: There are some updated thoughts here that I encourage everyone to read.