Grindelwald created a mind-link between him and Graves that allowed him to read Graves thoughts and feelings, so that it would be easier to impersonate the Director. When Graves is rescued, most of the link is dissolved, but Grindelwald used such a powerful charm that the medics can't get rid of it entirely.

After a while, Graves is captured by some other psycho (yes, he is unlucky, isn't he?) and tortured. Because of the link, Grindelwald can hear and feel it when Graves is tortured. This leads to him wanting to help MACUSA find Graves, and when MACUSA doesn't trust him enough to accept his advice, he breaks out to rescue Graves himself.