ADF "dip" is an antenna effect caused by the aircraft structure; it causes the ADF head to have an error (towards the lower wing) equal to approx half the bank angle. This is why the aircraft seems to be on the correct inbound track when you've still got 60 degrees of heading change to go, but how magically it all works out when you roll wings level.
It's pretty hard (and rather confusing) to fly decent NDB holds without understanding dip and there's no reason not to mention it during IMC training - it makes it easier (accepting that accurate holds aren't an essential part of the IMC rating).
HFD