Wikipedia
Style-shifting is a term in sociolinguistics referring to alternation between styles of speech included in a linguistic repertoire of an individual speaker. As noted by Eckert and Rickford, in sociolinguistic literature terms style and register sometimes have been used interchangeably. Also, various connotations of style are a subject of study in stylistics.
Style-shifting is a manifestation of intraspeaker (within-speaker) variation, in contrast with interspeaker (between-speakers) variation. It is a voluntary act which an individual effects in order to respond to or initiate changes in sociolinguistic situation (e.g., interlocutor-related, setting-related, topic-related).
William Labov, while conducting sociolinguistic interviews, designated two types of spoken style, casual and formal, and three types of reading style (a reading passage, a word list, and a minimal pair list). Analysing style-shifting Labov postulated that "styles can be arranged along a single dimension, measured by the amount of attention paid to speech" (1972, as quoted in), casual style requiring the least amount of conscious self-monitoring. Such style-shifting is often referred to as responsive (produced in response to normative pressures).
In recent developments of stylistic variation analysis scholars such as Allan Bell, Barbara Johnstone, Natalie Schilling-Estes have been focusing on initiative dimension of style-shifting, which occurs when speakers proactively choose among various linguistic resources (e.g. dialectal, archaic or vernacular forms) in order to present themselves in a specific way. In initiative style-shifting speakers actively engage in social practices to construct social meaning.