2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/189205The opening of a new S-wave threshold is frequently accompanied by an abrupt dip in the magnitude of an amplitude for an already-open channel. One familiar example is the behavior of the I=0 S-wave $ππ$ scattering amplitude at $K \bar K$ threshold. Numerous other examples of this phenomenon in recent data are noted, and a unified description of the underlying dynamics is sought.17 pages, 2 figures. Two additional references; typographic correction. To be published in Phys. Rev. DHigh Energy Physics - PhenomenologyHigh Energy Physics - Experimenttext