We are told that a slow takeout double suggests leaving it in, and a slow penalty double suggests taking it out.
On this auction:
What would you expect partner's double to be, and following from that, what do you think a slow double would suggest?
If you were told that double was neither takeout nor penalty, but 50/50 and partner is expected to judge what to do, what do you think a slow double would suggest?
If it matters, and I think it might, they were playing four card majors.