Sentence:  At caving conventions aboveground, cavers often squirrel themselves through adjustable wooden "<WSD>squeeze boxes</WSD>" in good-natured competitions. In the safety of this controlled setting, cavers may push their capacities far beyond what they might hazard underground; ... A very slender woman's tightest squeeze may be defined by the width of her skull turned sideways. Some women thus emerge from <WSD>squeeze boxes</WSD>, triumphant, with mirrored abrasions over their cheekbones. 
Meaning of the word marked with <WSD> tags:  A box with an adjustable opening used by cavers to practise crawling through tight spaces.