This does not mean that it is bad under all circumstances to make
a Middle-game move during the opening stage of a game. But only
such moves should be considered in cases of this kind as a player
is fairly sure to make at any rate within the further course of
the game with a view to increasing the mobility of the piece in
question.

This is the main point. A second move made with a piece must
improve its position, otherwise, common sense tells us, it is
surely bad. For instance: After (1) P-e4, P-e5; (2) Kt-f3, Kt-c6;
(3) B-b5, Kt-f6; (4) o-o, B-e7 there is no objection to White's
playing (5) R-e1 as the Rook will very likely want to get into
action in the e-file in any case, as soon as the development has
progressed with P-d4, Kt-c3, B-g5, etc.

But if in an opening like (1) P-e4, P-e5; (2) Kt-f3, Kt-c6; (3)
B-c4, Kt-f6 White plays (4) Kt-g5 for instance, or (4) B-d5, it
is evident that he merely wastes time, for in the first case he
places the Knight on a square from which he is sure to be driven
away again as soon as the direct attack involved in his move has
been warded off, and in the second case he moves the Bishop to a
square which does not afford him any more mobility than the one
on which he stood before.

As a rule only Rooks or Knights are in a position, during the
opening, to add to their mobility by a second move; the Rooks by
occupying a file which is liable to be opened by an exchange of
Pawns, and the Knights by occupying a square in the center of the
board.