When the author to the Hebrews tells them to "remember" those in prison, what does he mean? Are they supposed to say, "Oh, that's right. I forgot" (i.e., punctilear), or is he saying that they should constantly think of those in prison (i.e., imperfective)?

We used to teach that an imperative built on the present tense stem indicated an action already underway. This is most evident in prohibitions, for example, to tell a barking dog to stop. We know this is no longer the case, but it does surface from time to time in translations and commentaries.