expression, pragmatic marker
as a parenthetical expression, marking a metaphorical rather than literal sense.
In the sentence "He's a sadist, if you will", the phrase signals that the speaker is acknowledging that the choice of words might be unfamiliar or novel to the audience in that particular context. The expression is likely derived from a clipping of a clause such as "if you will allow me to express my idea in these words".
The construction is identified in Brinton (2008: 164) as Canadian by frequency in comparison to UK data (see Chart 1). However, comparison with newspaper data from Canada, the UK and the US for 2009 and 2010 (Chart 2), reveals it as a predominantly North American construction that is no Canadianism as defined in the DCHP-2 typology. The data in Chart 2 is roughly equivalent to the three pragmatic meanings in Chart 1. There may be a chance that some sub meaning may reveal a national dimension, but our current data does not support such a claim. Brinton's data, which is based on the Strathy Corpus (to 2005), shows that parenthetical if you will “is more fully grammaticalized as a pragmatic marker in Canadian English than in British English” (Brinton 2008: 164). The American data suggests that the construction is most grammaticalized in the US, with Canada on its heels and Britain lagging behind.
Images:

Chart 1: Pragmatic marker if you will by function in British English (BNC) and Canadian English (Strathy) (source: Brinton 2008: 164) 
Chart 2: Tokens of pragmatic if you will in The Globe and Mail, The London Times, and The New York Times in 2009 and 2010.