My trailer has "seide" (?) skids - probably misspelled "side" skids? If
it has a flat the axle is supported by the skid. I suppose the flat tire
indicator is the shower of sparks coming from the skid!! A severely low,
but not flat, tire would, when getting lower and lower, cause
intermittent contact of the skid with the road, therefore intermittent
showers of sparks. The skids do not appear to have ever touched the road,
so there must never have been a flat while moving.
Al G.