You are right, and it is a very good example of this power. I think it will work when the king (or queen) has the support of the people which Johnson would not have in that situation. The Danish king in 1920 knew that it was not just the politicians that would be enraged by his move, but the people. In fact, at the time, just after the Russian revolution, every politician feared a communist takeover.Overt power or influence is rarely exercised, but a lot goes on behind the scenes. A recent example is when Boris Johnson tried to prorogue Parliament. This required the assent of the Queen, which would have involved her in political manouvering. Quiet talks were held, and it was made clear to Bojo that the Queen could, if pushed, refuse his request, leading to a political crisis. Johnson was forced to back down, not by the use of actual power, but by the quiet threat of it.
The Spanish king Juan Carlos prevented a coup in 1981, and this was probably not because he had the support of the people, but rather because the coup was led by officers who professed an allegiance to the king.