Originally Posted by
davidad
What exactly causes the lowering of tension? Compression, maybe?
Well, the discussion so far has centered on tension on the spokes. To that end, deformation of the local part of the rim is the cause of local lowered spoke tension.
When tensioned, spokes stretch. When the rim deforms under load the small portion of the rim, near where the tire touches the road, is deflected and the hub closer together. This reduces the length between spoke hole and nipple, and the spoke tension for those spokes is lowered. They're still in tension, but less so.
The key failure mode for a spoke in compression would be buckling. It would take about a quarter kilogram to buckle a spoke. So no, no compression.
Put another way: less tension, but zero compression.