「Improbable, cannot do with vertical rod」plainly states something that male「rods are unable」to perform: MULTIPLE TIMES again and again, at least not without first resting during a refractory period…orifices of any sex, on the other hand, can. Needless to point out, but vertical rods also make lousy conveyance devices.
　Pundits say formerly perhaps both shapes were drawn more roundish, and could simply have depicted a wheel turning on an axle…but, why would anyone in imperial China care to count the rotations of a wheel, an incredibly insignificant and ubiquitous action? note that the source glyph (囘) was unrelated to wheeled carts, and included「snake, myself」「penis in orifice」「spiralling vortex」within「spread legs」and meant the same thing as this glyph, namely: exactly HOW MANY TIMES did that occur? taking turns most likely with the herd assets of a lord, from one sexual partner to the next, or in bonobo-style, orgiastic melees, clusters of folks, gang-raping (see 迴 and 廻) a ewe or human, and so on, including the shaman's COUNT of her sex slave properties providing billable services.「Penned-in, enclosure of orifice」is something or someplace that one cannot escape easily, such as when both feet have been no-longer-integrated and pulverized, and permits stuff such as a「contained orifice with vertical rod」MULTIPLE TIMES, RETURN, GO BACK AGAIN, and so on: a SEX SLAVE, with the same connotations as 囘,「snake inside spread legs.」
　ANNULUS is the area between two concentric circles, a ring-shaped object such as a COCKRING that also enables one to perform MULTIPLE TIMES, RETURN, GO BACK and DO IT AGAIN and mitigates somewhat the condition described by the elements as「improbable with vertical rod」when placed into service without this tool, enabling「rod within spread legs with orifice, array of vagina」in MULTIPLE OCCURRENCES.

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: variant of 迴|回; to curve; to return; to revolve; to circle; to go back; to turn around; to answer; to return; to revolve; Hui ethnic group (Chinese Muslims); time; classifier for acts of a play; section or chapter (of a classic book)
Japanese: counter for occurrences, counter for games, rounds, etc., counter for innings, Islam, Hui, mawashi, belt, loincloth, mantle, cape, gang rape, to turn, to rotate, to gyrate, to circulate, to send around, to surround, to put something to a new use, rotation, circulation, to revolve, to visit several places, to function well, to pass a certain time, to return, to concern, to wander around, what goes around
Unihan extended: return, turn around; a time
EDRDG: -times; round; game; revolve; counter for occurrences