What makes a mand different from a command? According to Amanda Reed, BAppSc, MA, Director of Development at Pyramid Group Management Services Corporation (the creators of PECS picture cards), "A mand is essentially a request, but it's all about what comes before and after the request. Prior to the mand comes some kind of deprivation or aversive."
For example, a therapist, knowing that a child particularly likes Oreo cookies, might hold an Oreo in her hand and show it to the client. This is the deprivation or aversive. When the client correctly uses a mand by requesting the cookie using words, picture cards, signs, etc., the therapist responds by handing over the cookie. If the client simply grabs, the therapist withholds the cookie and instructs the client to use the appropriate mand.
Interview with Amanda Reed, BAppSc, MA, Director of Development at Pyramid Group Management Services Corporation. September, 2010.