Obviously my Aspergers means this bothers me more than most but one of the answers to the question "What behaviour did one MNer use a LEGO® set as a reward for?" is "Stopping to bite their fingernails". So she is rewarding them for stopping [whatever they are doing in order] to bite their fingernails? I'm willing to bet she is rewarding them for stopping biting their fingernails (or even better for NOT biting them)