the principles and practice of fly and bait casting

Is an excellent book by Reginald D. Hughes (published 1924). As usual I'm going to quote from a few pages:

The great majority of fishermen are ignorant of the actual principles which underlie their every act, whether right or wrong.

One certain index of efficiency is the absence of effort. Style is synonymous with efficiency, and style and effort do not go together.

Double-handed fly casting may especially be recommended as almost, if not quite, the most beneficial form of exercise one can take. It calls for the use of every muscle in the body. It exercises without exhausting.

Excessive effort is not only uncalled for, but if practised defeats itself.

Brute force alone will never put one in the front rank.

A tight grip means tense muscles and joints throughout the body. It kills all attempts to cast smoothly and easily. It imparts, through a vibrating rod tip, waves and irregularities to the line, and is very tiring even to an onlooker.

Remember that the rod should be practically noiseless. Any distinct "whoosh" is a sign of a faulty casting, and shows that the cast is made with the entire rod instead of with the top.

Hold or grip of the rod - there should be none. The rod merely rests in the right hand, while the left hand lightly encircles the butt end. Any tendency to a tight grip must be resolutely suppressed.

Both hands must do an equal share of the work.

Our desire is to cast the fly across and downstream at an angle of about 45 degrees. So we stand, as regards our feet, facing this direction, and, without moving them, rotate the body until we face downstream, rod low and pointing in the direction of the fly.

In learning these casts try to avoid too much concentration, as the great secret is to let the whole body be free and swing easily and comfortably, letting the rod do it, and it will do it if the timing is right.

If the line is allowed to slacken in the least, even momentarily, the pull on it is lost.