「Buttocks or genitals taut」as WHAT IS BETWEEN ONE'S THUMB AND MIDDLE FINGER…perhaps needs no further explanation, also as in to MEASURE SOMETHING PULSATING or THROBBING.
　「Genitals, buttocks, corpse」「taut, leashed」to another that is tied by rope or chained to another, and so on, depicting prisoners SHACKLED IN A LINE, a LINE-UP, or a LINE OF TIED GENITALS (see 縲 that describes the series of prisoners in line, 睪, 鈬, 械, 型, 形 and 矴 as an 'anchor, grapnel' using elements 'male stones'). This glyph also means EVENLY SPACED「dead men walking, to line-up slaves in equally MEASURED units」(to maintain a certain distance spacing of about ONE FOOT) and appears as an element included in many other glyphs. Tying prisoners or slaves by their genitals, if males, made the entire process of managing the group much easier, because in order to escape both the front and back prisoners would either fully cooperate or risk having their own genitals forcibly pulled off, all the way down the line…everyone needed to move in unison, or not move at all.
　Note that with something tied tightly around the base of one's scrotum behind the testicles, blood will normally flow into but not fully out of the genitals, causing involuntary erections until the rope is untied (this is exactly how a cockring or elastic band functions to maintain an erection). A line of such prisoners in motion would be a source of comedic relief for the guards, no doubt. Replaces glyph 睪 that means「guard closely, keeping close watch, the scrotum anus and schmekel region」or「netting of groin, night soil, torture of slaves or prisoners.」Shackling them into lines might make constantly guarding and watching a bit easier (and, the glyph is easier to write).
　Dictionaries suggest the old glyph means「spying on」yet the simplified Chinese now uses「talented two hands in crotch, rod changing state」(择) to represent this glyph, elements that do not relate too well to spying except by a peeper, and better relate to torturing someone by inserting needles into the anal orifice. In all fairness, the old glyph may also have referred to a physician rendering care to a patient with a serious crotch infection, using needles to draw out the accumulated festering pus.

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Chinese: a Chinese foot; one-third of a meter; a ruler; a tape-measure; one of the three acupoints for measuring pulse in Chinese medicine; one of the characters used to represent a musical note in gongche notation, 工尺譜|工尺谱
Japanese: distance between outstretched thumb and middle finger, ruler, measure, shaku, scale, length, small amount of silk, unimportant painting, trivial work of art
Unihan extended: Chinese measure approx. 'foot'
EDRDG: shaku; Japanese foot; measure; scale; rule

Used within names (nanori)
せき