My definition of a foreshaft is an unnecessary contrivance designed to make arrows weaker and more complicated.The same can be said of cut-in arrow shelves (on bows) and notches in arrowheads.
There is only one thing to say in response:
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Here's a series of pics that briefly shows the process for making and installing a foreshaft.
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An arrowhead of stone (or some other material besides wood) can be installed on the tip of the foreshaft....or the foreshaft can double as the arrowhead if left as-is. Foreshafts are usually installed on a lightweight arrow material (like cane, reed, or softwood) to give weight and strength to the tip without adding excess thickness. Most foreshafts that I have seen (on very old arrows) are permanent and do not come off the arrow after shooting. It is very similar to a "footing" on an arrow.
Hope that helps. I'm sure other will chime in.