I think the idea of an instructor surprising a student is not a very good instructional technique. If I wanted to demonstrate the DG-1000 behaviour that you describe, I would brief the student and then demonstrate it or if the student was reasonably competent, have him do it.
I'd agree and I'm at fault for not being sufficiently clear - by student, I meant P2 on our biannual refresher training. So I guess the issue for the instructor is 'how do you assess P2's competency in spin recovery if P2 is the one being asked to spin the glider in the first place'. I admit to being taken aback the first time it was practiced on me. I think the advantage of this instructing technique is that P2 has to actively think about the use of contra-spin rudder rather than practice the reflex of simply applying opposite rudder as is normally the case when the spin is P2 initiated.
I'm sure you'll enjoy flying the K21 and DG-1000. Both great flying machines (as is the Duo).