In Toaq Gamma, the hook above diacritic of 
(the "verb tone") was very common. Sparse tone marking was a way of writing Toaq without this diacritic. For example, Chẻo jảq chỏ ke pảı da was written Cheo jaq cho ke paı da.
Essentially, every word was written as if it was in 
(the neutral 8th tone, reserved for particles). Since verbs and particles are different lexical classes, this didn't cause any ambiguity. If you saw a verb in , you knew it was actually an implicit .
Perhaps inspired by this practice, the modern falling tone 
is denoted by the absence of a diacritic. See Tones.