The yaw string just indicates the direction of the flow in the boundary
layer at that point. I have seen in a stabilized spin in a 2-32 the yaw
string point in the direction of the turn, straight forward, straight
aft and in a direction opposite the direction of the rotation. This was
an induced spin with the CG almost at the max allowable aft position. I
have heard reports of an ASK-21 doing similar things but have not
experienced it.