In “The roots of Old Chinese” (Sagart 1999: 132, fn. 1), I described the shangsheng 上声 tone in the dialect of Xiaoyi 孝义 in Shanxi as being “characterized by a glottal break in the middle of the syllable [31ʔ12]”. Ho (2016:186) argued that “the presence of a glottal stop is due to the concave tone (312)”. He added: “it is quite common in Chinese dialects to observe a breaking point in the lowest point of the contour tone. This is witnessed in the Beijing speech as well”. Spectrograms and audio files for three syllables: 草，普 and 委, are provided below to illustrate the extent of the phonetic difference between the creakiness at times occurring at the lowest point of the Beijing shangsheng 上声 tone and the longer, controlled interruption of voicing in the Xiaoyi 孝义 shangsheng 上声 tone.
Ho Dah-an. 2016. SUCH ERRORS COULD HAVE BEEN AVOIDED—Review of Old Chinese: a new reconstruction. Journal of Chinese Linguistics 44 (1). 175–230. doi:10.1353/jcl.2016.0004.
Sagart, L. (1999). The Roots of Old Chinese. Current Issues in Linguistic Theory, 184. Amsterdam: John Benjamins.