Udarka Mark

It marks the first sound as the voice drops.

A 1910 primer describes a swift fall from a higher pitch to a lower one, with that first sound struck. A neume is the chant sign for one or more notes sung on a single syllable; Udarka modifies that sign rather than naming a pitch by itself.

How did Znamenny notation develop?

It grew from an early Byzantine chant notation system.

Znamenny chant is Orthodox liturgical singing. It developed in Russia in the 11th century from an early Byzantine musical notation system. The earliest surviving Type C manuscripts, from the 15th century onward, have no pitch marks. Type B, introduced in the first half of the 17th century, added red-ink marks called cinnabar marks to show pitch and dynamics; Udarka belongs to this class. Znamenny chant was the predominant form of liturgical music in Russia and Ukraine until the late 17th century. After that, Russian Old Ritualists and some Russian Orthodox monasteries and parishes continued it.