MI – A board stretcher is a thing you tell to new inexperienced workers in a wood shop or whatever. If they cut something too short you go “Oh! You cut that too short! Go get the board stretcher!” to go stretch the board back out to the right length. And they go looking for it but it doesn’t exist. It’s like a snipe hunt. Everybody gets a good laugh while the new guy makes a fool of himself.
This is an example of a snipe hunt, as mentioned by the informant. It is not possible to stretch a board back out to the right length when it is cut too short. But a new employee, probably worried about making mistakes and cognizant that they know much less than everyone else in the shop, will eagerly listen to the more experience workers even at the expense of their own logic. The practical joke played on the new employee will possibly show them, through humor, that the older employees are not upset at them for making a mistake. As snipe hunts can only be played once per person, the new employee will then become more experienced in the wood shop culture, and therefore takes another step into the in-group—or folk group—of the wood shop.