What it measures
Every club is built with a lie angle, the angle between the shaft and the ground when the sole sits flat. Dynamic lie is that same angle as you actually deliver the club at impact. It is measured along the lower part of the shaft because the shaft bends during the swing.
A higher number than the club's built lie means the shaft was more upright at impact, so the toe was up and the heel was down. A lower number means the shaft was flatter, so the toe was down and the heel was up. The reading is the same for right and left-handed golfers. For reference, built lie angles are roughly 56 to 60 degrees for drivers, about 62 to 63 for a 7 iron and about 63 to 64 for wedges, and they vary by maker.
Typical numbers
There is no useful tour average for dynamic lie, because what matters is how your number compares with your own club's built lie. A common pattern is a reading slightly flatter than the built lie, because the shaft bows toe down under the pull of the clubhead in the downswing. That droop can be anywhere from a fraction of a degree to around two degrees, more with longer clubs, heavier heads, softer shafts and faster swings. Woods, with the longest shafts, tend to show the most droop. Golfers who stand up through impact often read more upright than their clubs.
What changes it
Hand height at impact is the main driver. Hands that rise above where they were at address, usually from the hips moving toward the ball or the chest lifting, make the reading more upright. Hands that drop lower, or a golfer who reaches for a ball set too far away, make it flatter. Shaft flex, head weight and swing speed change how much the toe droops, and the built lie of the club sets the baseline you compare against.
What it changes
A lofted face that is tilted toe up points to the left of where it would point with the sole flat, and the spin axis tilts left too, so upright delivery produces pulls and extra draw for a right-handed golfer. Toe down does the opposite and sends the ball right. The effect grows with loft: two degrees too upright turns a 7 iron face roughly a degree left, a wedge face about two degrees, and a driver face only a fraction of a degree. Off the mat, the low side of the sole also catches the ground first and can twist the face.
Dynamic lie also ties into strike location and swing plane, because the same hand movement that raises the plane through impact usually raises the lie.
How to use it
Find the built lie of your clubs, then compare your average over a group of solid strikes, not one swing. If you consistently read a couple of degrees off with a particular club and miss to the matching side, a fitter can usually bend most irons by about two degrees either way. Fix delivery first if the number only goes wrong on poor swings.
Bay drill: put face tape or a dry-erase line on a wedge face and hit ten shots, noting dynamic lie, strike location and start direction together. If upright readings line up with heel strikes and pulls, rehearse keeping your chest down and your hips back through impact, then check whether the lie, the strike and the start line all move back toward center.