The answer is taut. Milady states this directly: taut skin allows the beard hair to be cut more easily, which is why the nondominant hand's job during a shave is to gently stretch the skin.
The book is specific about what "taut" means here, and warns against the neighboring extremes: loose skin tends to push out in front of the razor and can result in cuts or nicks, while stretching the skin too tightly causes irritation. Taut is the correct middle ground, not simply "tight" or "dry."
Milady Standard Barbering, 6th ed., Ch. 13 (Shaving and Facial-Hair Design), p. 348: "While the right hand holds and strokes the razor, the fingers of the left hand gently stretch the skin area being shaved. Taut skin allows the beard hair to be cut more easily."
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