From Wikipedia entry on the Bene Gesserit sisterhood:
The Bene Gesserit are notable for their extensive skill in seduction, sex and sexual imprinting. The most talented and most highly trained are known as Imprinters. Men in a position of power or future power, or those with specific qualities that the order wishes to incorporate into their breeding program, are typical targets of a Bene Gesserit imprinter. Men seduced by an imprinter are permanently affected (imprinted) by the intense sexual experience and are thereafter consciously or subconsciously favorable to the Sisterhood. An imprinter can be successfully resisted if the subject has been psychologically pre-conditioned to do so, and the subject's automatic defensive response may even be entirely subconscious.