What it measures
Dynamic loft is the upward tilt of the clubface, in degrees, at the spot where the ball touches it, at the moment of maximum compression. It is measured against the horizon, not against the shaft or the club's built in loft.
A 7 iron might be built with 30 to 34 degrees of loft, but if the hands are ahead of the clubhead at impact the face is tilted forward and the delivered loft is lower. If the clubhead passes the hands, or the face is open, it is higher. The number is always positive for normal full shots, and the meaning is the same for right and left handed golfers.
Typical numbers
PGA Tour players average about 12.8 degrees with the driver and about 20.2 degrees with a 6 iron. LPGA Tour players deliver more, about 15.5 degrees with the driver and 23.6 degrees with a 6 iron, which helps them launch the ball higher at lower speeds. A tour 7 iron sits a little above the 6 iron, in the low 20s, and full wedges deliver well into the 30s or more depending on the club and shot.
Everyday golfers usually deliver more loft than tour players. Male amateurs average from about 13 degrees with the driver for scratch players to about 15 degrees for the average 14 handicap. With irons, many amateurs add loft by letting the clubhead pass the hands, which costs distance and makes the ball balloon.
What changes it
Shaft lean is the main factor. Hands ahead of the ball at impact lower dynamic loft, hands even with or behind the ball raise it. Attack angle is linked: hitting down tends to come with more forward lean and less loft, hitting up with the driver usually adds a little.
The face's rotation also matters. A face open to the path tends to show more loft, a closed face less. Strike location changes it on woods because the face is curved: a high strike on a driver adds loft, a low strike removes it. Shaft bend at impact, ball position and the club's built in loft all feed in as well.
What it changes
Dynamic loft is the single biggest driver of launch angle. The ball launches a little lower than the delivered loft, about 80 percent of it with a driver and closer to 70 percent with mid irons, based on tour averages. More loft means a higher start.
It also sets spin. Spin loft, roughly dynamic loft minus attack angle, controls how much backspin the ball gets, so extra loft without a matching change in attack angle adds spin. Too much dynamic loft gives high, weak shots that lose distance, especially into wind. Too little gives low launch, low spin, and shots that fall short and run out.
How to use it
Compare your number to the club's built in loft. If your 7 iron is consistently near or above its stated loft, you are likely adding loft through impact, and learning to lead with the hands will add distance and lower your flight. If your driver loft is very high and spin is high, a slightly forward ball position with less added loft, or a club fitting, may help.
Bay drill: hit five 7 irons normally, then five while making a small rehearsal with the hands slightly ahead of the ball before each shot. Watch dynamic loft, launch and carry. A drop of a few degrees in dynamic loft with similar or better carry is a sign of a more efficient strike.