Not checks in which the amount is blank. Checks in which the name in the “To” line is not filled out at the time, and is later filled out by the guy he gives the check to.
Suppose a guy writes such a check and the guy he gives it to fills in the name of his drug dealer, or the name of someone he is dealing with in some financial scam. Is the first guy on the hook for anything? Is the fact that he signed the check that is made out to so-and-so and was used for such-and-such transaction prima facie evidence that he was involved in the deal? Or can he get away easily with his (true) claim that he gave the first guy a blank check and has no involvement with the later use to which the check was put?