When the swing is initiated, the shaft is loaded. Someone with a fast tempo will aggressively load the shaft requiring a firmer flex than someone with smooth acceleration to achieve the same kick through the ball. In addition, swing speed can increase with the proper flex or decrease with improper fit. The only way to find the right shaft is to hit the club and evaluate the feel. There are really no numbers to distinguish a standard for "reguar" and "stiff" across manufacturers. The closest comparison scale would be CPM or Cycles Per Minute. In this way, an exact number for each shaft can be recorded and each flex designation can be compared and the proper fit can be achieved. So this is all old news to you... OK no worries... but we're here to talk about why this is important.
Many studies have shown that swing speed decreases slightly before impact. It's a byproduct of physiology for a normal swing. The AMOUNT that it decreases differs depending on where that shaft is around the impact area and how you release your hands. Shafts flex and re-flex during the swing. Usually, with a driver, that shaft is flexing towards the ball slightly and is releasing all it's energy into the ball through the clubface. This is done by "Braking the swing" or "throwing the club" as some teachers will describe it. You may have also heard "hitting against a strong left side (right handed)". Explanation is for right handed golfers. Just flip it for lefties - What this means is that as the swing motion happens, the left leg becomes the pivot and essentially puts the brakes on the swing, allowing the arms and wrists to create MORE speed as the energy transfer moves through the grip and shaft and into the ball. The best way to illustrate this is to use a trebuchet - a medieval device used to hurl large objects very far away. "But why not a catapault?" you ask? Because a catapult is a fixed fulcrum and just using the flex of the shaft to hurl the ball. The trebuchet has moving parts that perfectly mimic the golf swing --- well, if you turn it upside down. Watch below it's the best to illustrate what i'm talking about. While you view, imagine the wood arm of the trebuchet is your arms and wrists, the rope is the shaft of the golf club. Notice, as the energy peaks, the trebuchet arm STOPS but the rope keeps moving and releases the stored energy (and the rock) ultimately hurling the object 100s of feet. Turn it upside down and you'll realize that this STOP point is roughly the same as in a golf swing. Your arms just keep moving forward because let's face it - if you stopped a full golf swing like that, your shoulders would probably dislocate and you'd blow out a knee.
The next video is this notion in practice. Long drivers, and Dustin Johnson being the best example. Notice the lag of the club and right at the bottom, the player puts on the brakes with the left leg, throwing that club through and that poor ball can't do a thing about it. Notice the leg straightens and his foot even moves back a little bit. the hips PAUSE then keep moving through. It's an upward motion to stop the swing, very much like (if you turn this video upside down) the downward motion of the weights in the trebuchet. If you flip the top video upside down, the mechanics are the same.
OK so what does all this have to do with shaft flex? Well, a proper swing is how you get the shaft to flex and a proper swing is how you get the shaft to transfer power. If this is something you can't do - bad knees, bad back, just slow speed in general -- you need a shaft that will load and unload easier and help to get the maximum performance out of your club. Having a shaft that is too stiff means that even if you're aggressive in loading it, you may not be able to have it UNLOAD properly since you have to maintain that speed and put the brakes on aggressively. That stiffness will cause it to unload the power early - robbing you of distance. Same with a shaft that is too flexible. At the point where the brakes go on, it may have way too much energy and will not unflex efficiently to transfer that power, resulting in loss of distance and accuracy.
So what does all this mean? Well - OEMs make "whole flex" shafts as I like to call them. R, S, Firm, X, etc. They have no interest in getting particular with a shaft because it doesn't fit the market. They want to sell as many clubs to the widest array of people. An R flex will most likely work for you, but between R and S there are a multitude of flexes. Maybe removing 1/2" from the tip is a perfect stiffness for you. Maybe your optimal is JUST UNDER R flex? Just Under or over Stiff? Maybe it's just adding 1 swingweight point to your driver, making it the perfect flex and weight. Remember, lighter isn't always better and faster.
See a competent fitter and hit some different shafts. Even message me for some brain-picking if you want. The difference between hitting more fairways and hitting longer drives doesn't simply rest in a new $500 driver. It may be just taking that old trusty club and adding a little tweak. Use the money you save for beer.