I typed this response to Ian's post below about 'sheetgoods bending when ripping'.
I certainly would not ever want to hijack anybody else's thread so I thought I would proffer this in it's own post.
It is based on purely anecdotal science so could also just be the movie in my own head. Maybe Gene Wengert could weigh in this if he should so desire.
In the beginning.................Wood is actually a series of straws. The straws constitute arteries that connect each leaf with nutrients in the soil, These straws are arranged in a circle that emanates from the center of the tree. The tree undulates as it grows and each plank represents a different slice of that circular undulation of straws. (rift cut, plain slice, quartersawn etc)
During the lifecycle of the tree it sometimes has prosperous seasons and sometimes has to struggle for nutrients. Trees that grow in a young forest grow quicker because there is less competition for sunlight and moisture. Trees that grow after a forest fire get different juju out of the soil than trees that grow on managed plantation forests. As the forest canopy increases the growth rings get tighter (straws get smaller. (Imagine this as a symphony).
I am not sure about why this banana cut happens to sheets cut with a saw but not with a router. I suspect, however, that it might have something to do with heat.
Watch what happens when you hand feed a piece of lumber through a table saw. The lumber itself stalls ever so slightly while your hands are changing position during the rip. Sometimes the fibers of the board collapse in a way that binds the lumber onto the blade. When this happens you can sometimes get a burned edge on the side of the board. In extreme cases you can actually see smoke come off the board.
What is happening here is that the lumber is actually catching on fire. That is a LOT of heat!
Sometimes the tension release manifests in the opposite direction. Rather than pinching onto the blade the saw kerf gets grotesquely wider at the cut end as you are pushing through the blade.
The tension that causes boards to twist while they are being cut has sometimes to do with species or how the plank was cut. Sometimes it has to do with moisture intrinsic in the lumber. Sometimes it is the sharpness, toothset or cleanliness of the tablesaw blade.
About 20 years ago someone told us to try ripping lumber on a bandsaw rather than a tablesaw. They said we would get straighter boards out of the saw than if we ripped them on a table saw. This turned out to be true.
It used to be when we were building cabinet doors we would always allocate for the tall doors first. If we needed four 60 inch stiles we would start with at least eight pieces then pick the straightest ones and use the extras for smaller needs. As soon as we started doing initial ripping with a bandsaw, however, the need for extra planks was significantly reduced.
As it was explained to me, the cutting action of a bandsaw approaches the lumber fibers 90º opposite from that of a table saw. In this respect the cutting is more like how it is when you crosscut fibers with a miter gauge vs ripping with a fence. The cutting influence is not parallel with the fibers but opposite them There is no opportunity here for the wood fibers to seize on the blade and each tooth of the blade has a chance to cool down slightly as it rotates through the spin cycle.
The heat is easy to observe. What is less obvious is the influence of this heat as it is imparted to the woodfibers just at the location and point in time as the intrinsic tension of the plank is being modified by the blade.
Think for a second about what happens to lumber as it is being milled. If you take a perfectly straight & flat board and rip it down the middle one or both resulting planks is no longer straight. This has to do with the intrinsic tension in the board.
Try it for yourself: Rip 10 boards with a tablesaw and then rip 10 similar boards with a bandsaw and see how generally straight the sticks are.