The King in the Net.
True, the Black K on his QB5 cannot get over the Q file nor his fourth row as long as the White Rook, protected by its King, stands on guard on Q5. But how is the net to be drawn tighter? If the Black King should refuse to move, the tightening of the net would be impossible.
The fish in the net of the fisherman might refuse to move, not so the King in the net of the Rook. The right of moving in Chess is at the same time an obligation. In by far most instances the right to move is of great value, but there are cases, as shown above, where to move is disadvantageous. Yet, rightly or wrongly, the laws of Chess do not permit a free choice in this respect: you have to move, whether you like it or find it irksome. The constraint to move is usually called by a German word "Zwgzwang," that has become international in its usage.
Let us return to the above position, White to move. White plays King from K6 to Q6. If Black replies King from B5-X5, or X6, the Rook will follow it up with R-QB5, thus cutting off the QB file; and if Black replies King from B5-B6, the White King will get into Opposition, King from Q6 to B5, and the play might further proceed methodically King from B6 to X6, R from Q5 to Q4, King from X6 to B6, King from B5 to Q5, King from B6 to B7, King from Q5 to B4 and so on. To drive the King by means of the _Zugzwang_ on to the border and there to give it the Checkmate is, after all that has been said, an easy task.