The Harrison Shotmaker is a USGA-approved removable insert which goes into your driver shaft to improve accuracy. The tip section of a golf shaft is the most flexible area of the shaft and as such it tends to deform and vibrate upon impact, and this inconsistency affects driver accuracy. The Shotmarker – a strong, lightweight graphite insert - strengthens the tip section of the shaft, and by stabilizing this area your shot accuracy will improve dramatically!
Choosing the Correct Flex
To determine the proper "flex" for the insert, it is important to look at both the clubhead speed and the load factor of the golfer. Clubhead speed can be measured for you very easy at your nearest golf shop. However, if you don't have a way to measure your clubhead speed then you can estimate it based on average driving distances for various swing speed ranges:
<80mph (190 yards or less)
80-90mph (190-220 yards)
90-100mph (220-250 yards)
100-110mph (250-280 yards)
110-120mph (280-310 yards)
120-130mph (310-340 yards)
130+ mph (340+ yards)
The second thing you need to consider is the shaft load. In a nutshell, the fasteryour swing tempo is (tempo is the amount of time it takes for you to complete the swing), the more load you will put on the shaft during your swing. So if you have a very slow smooth tempo then you will want to choose the flex at the low and of your swing speed range; if you have a moderate or medium tempo then choose the one in the middle; or if you have a very fast aggressive tempo then choose the one at the high end of your swing speed range.
Below are a few videos which show different swing tempos. PGA Tour player Ernie Els has a longer swing arc with a smoother tempo, whereas Sergio Garcia has a very short swing arc with an faster tempo. If your swing is similar to Ernie then you have a slower tempo, whereas if your swing is similar to Sergio then you have a faster tempo.
Ernie’s Golf Swing: Video
Sergio’s Golf Swing: Video