BNK is an acronym for Brains no kiss. This is to achieve the act of felatio/cunnilingus without reciprocating in any way.
Softly caress a girl and hint towards your crotch subtly. Use a slight directional head nod. Do not kiss her but maintain physical contact via long senuous stroke from the back of her neck to her butt. If she/he give you brains, you have just achieved a BNK