This study examines the simultaneous use of English and Chinese by speakers of Chinese as a heritage language (CHL). It focuses on spontaneous, dynamic, and high-density mixing of the two languages within the smallest building block of a speaking turn: the turn constructional unit (TCU). Drawing upon data from different age and proficiency groups, it shows that CHL speakers migrate between and mingle the two language systems freely and frequently at multiple and nested levels of phonemes, morphemes, syllable structures, tones, noun and verb phrases, and clauses. It reconsiders existing syntactic descriptions, sociopragmatic taxonomies, and conversation-analytic accounts concerning codeswitching and concludes that rather than being governed by fixed, a priori linguistic constraints, sociopragmatic meaning, or conversation-sequential contingency, this type of ad hoc, intra-TCU codeswitching is accomplished with a full range of verbal resources from both languages that are simultaneously accessible to the speaker and might be accounted for by a type of rationality characterized by fuzzy logical conditions. It further suggests that, pushing and breaking perceptible linguistic boundaries, CHL speakers celebrate their multi-competence, traverse invisible cultural and identity boundaries, and employ and enjoy both languages at all ages and proficiency levels in transient and transcendent "multi-performances," which in turn can become a rich resource for heritage language socialization across the lifespan. (Contains 1 table.)