Every jigsaw I've used had an adjustment knob with an illustration that looked like back-and-forth "wobble" in the blade.
What is this adjustment used for? I've always just set it as what I figured was "least wobble"
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It has to do with how far back the blade goes on the down (non cutting) stroke. It's usually called the "Pendulum Stroke adjustment." The idea being that it will move the blade back, out of the way of the material on the down stroke.
It reduces the load on the saw when cutting thick materials, at the cost of a bit more splintering. Use a setting of 0 for fine finish work, and go slow, and 4 for more aggressive cuts where a bit of surface splintering isn't an issue.