The Sime's whole body is laced with nerves sensitive to the surroundings fields generated by the Gen body. Via the energy fields emitted by Gens as a side-effect of producing selyn, the Sime zlins the local surroundings. Gens act like a lamp acts to the eye-sight.
Simes are depressions in the ambient like this:
Gens are peaks -- just visualize that depression upside down.
Put a whole lot of Simes and Gens in a confined space, and average the resultant grid pattern.
As a moving electron generates a magnetic field "at right angles to" the electric field generated by the electron's path through space, likewise selyn in motion generates a selyn field and at right angles to that selyn field, a nageric field that carries emotional information and everything a Sime has to know to hunt Gens for the Kill.
Here's a video about electricity and magnetism and magnetic force that makes electricity simple.
You can now imagine the same video aimed at First Year channels making field management simple.
But it can be visualized best in this ultra-simplified form of the electromagnetic wave propagation.
Ancients can't sense electrical fields directly, but Simes can sense selyn fields directly - and Gens can not sense selyn fields directly.
Gens can not "read" selyn fields the way a Sime can - but the higher order Donors who are extremely sensitive, can learn to become aware of how their own body responds to selyn fields in flux. The human brain can learn to interpret such information so that it seems as if the Gen is "reading" the fields - but the Gen is not doing what the Sime can't help but do.
Thus when orgonics technology becomes the basis of modern living, and of the powering of starships, only Simes can service that equipment without danger of death by shock. A few, extraordinary and tremendously valuable Gens with a quirkly, technical bent of mind do learn to maintain orgonics equipment, but for them it's more a hobby or a bizarre challenge than a profession. Such Gens are top level Donors.
Nageric energy does not penetrate inanimate materials well, and is stopped by densities such as rock, the bulk of a planet, or metals such as lead. Some arrangements of dense materials can create effects such as a diffraction grid pattern in a nager, or viciate the intensity by scattering the energy so that a point source that would seem excrutiatingly bright is diffused into a lambent glow. Some such effects are pleasant to zlin -- others not so much.
Selyn Insulation originated as the use of solid rock. Gens could hide from Simes in caves, at the bottom of canyons, behind rocks or hills. But not in wooden cabins or other structures.
Wood can be zlinned through, but it does attenuate the field strength of the selyn energy field. Eventually, artificial materials are created by Simes (not Gens) that block nageric field propagation.
Simes are motivated to control local selyn field effects for personal comfort, but it is the channels who truly require materials that can insulate from local fields so that the channel can administer to the requirements of renSimes and Gens during channel's functionals, and find respit from the hammering of need, pain, and emotional storms.
So the Householdings make the first advances in selyn insulation. Zeor is particularly famous for chemistry and materials science. Insulating screens and wall treatments become marketable to junct society, providing all Householding hallmarks are removed. Junct society considers the Householders to be perverts.
Eventually, some advanced materials can be extruded into thread-like flexible forms and woven into other materials such as tent canvass, or hospital curtains, to insulate Simes from a local ambient nager. Gens who do not associate with Simes have little use for such expensive materials.
Psychospatial Orientation is a separate sense from the ability to zlin. This is the sense that Simes (but not Gens) have of exactly where they are in the universe at any given moment in time.
Together with the Sime sense of the passage of time, which is related to Orientation, this Sime perception allows Simes to become the astrogators of the Galaxy.
Here are some images of the crazy-corkscrew ride we have on the surface of the Earth throught time.
The sun moves through space orbiting the center of the galaxy, which is moving through space, too. The Earth is dragged through space following the Sun on a spiral course. And of course, the entire universe is "expanding."
Simes know exactly where they ARE in an absolute coordinates. Nobody knows what the graphic "origin" of that absolute coordinate system is, but all Simes orient on the same absolute location.
The Sime sense of location is the sense of elapsed-time which is tied to the ideosyncratic and precise sense of Need in the Need cycle. To Sime perception, Time, Space and Life itself are one single, inseparable and incomprehensible thing. No Gen ever attains that perception level, not even the Endowed Gens (though it may seem to such Endowed Gens that they do.)