sinking / synching are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
sinking: (noun) a descent as through liquid (especially through water). (noun) a slow fall or decline (as for lack of strength). (noun) a feeling caused by uneasiness or apprehension.
synching: NA
sinking: s·IH·ng·k·ih·ng
synching: NA
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)