Swingweight is a measure of the relative heaviness of the head at the end of the lever that is the shaft. Add 3 or 4 grams to the head and the swingweight goes up by one point.
Flex designations like R, S and X are pretty meaningless. Hence the use of shaft frequency as a way to describe flex. In a frequency matched set, there is a frequency difference of roughly 4 cpm between adjacent clubs in a set. (e.g. 5 iron is 316 cpm and the 6 iron is 320 cpm) If you plot frequency vs club # (shaft length) you get a straight line sloped at 4 cpm. Yet the set is described as having x-stiff or 6.5 shafts. The frequency is affected by the length of the shaft and the weight of the head. If you make the shaft one inch long or add weight to the head to change the swingweight then the frquency of the shaft is no longer x-stiff.
If you cut an existing shaft down and don't change the headweight, the frequency will go up about 4 cpm per inch IIRC. However, the flex of the shaft will still be on the same sloped line and hence will still feel like the same stiffness.
If you add weight to bring the swingweight back up, then the frequency will decline leaving you with a slightly softer shaft.
So I agree with A.G. up to that point. I disagree on the last point, If you add enough headweight to make a driver swingweight at D10 then the frequency of the shaft in that configuration will be lower, and it will flex at something less than an "R".
And to add to the dorkiness, I believe it is a matter of physics that a 3 wood is straighter than a driver, just as a nine iron would be straighter than a 3 wood. There are a couple of reasons. A driver has less loft than a 3 wood. The lower the loft, the more sidespin relative to backspin is applied for each degree the clubface is open or closed at impact. The more sidespin, the more curvature. Also the amount of curvature depends on the speed of the ball. The higher the speed, the more the curvature. Ball speed is higher with the driver than the 3 wood, hence more curvature.