
Cats ears listening left and right while
drinking - Photo © tanakawho
Timmy our stray
cat is very alert to danger. When he eats his ears are more or less
permanently directed behind and to the side to pick up approaching
danger while he is occupied stuffing his face

.
The cat in the picture, above, is doing the same thing. Cats ears are
wonderfully expressive and useful. Not only do they hear a lot they
provide signals to other animals. Have a look at the
tamed
wild cat, the Serval,
for an example of the pattern on the ear which is turned towards other
animals to frighten them as the cat is signaling aggression.
The origin of the "back of the ear pointing forward" posture is that it
is between the defensive flattened posture and pricked forward posture.
In short it means ready for trouble. The ears can be flattened quickly
from this position. The signal to the other animal is "ready to attack".
There are apparently 5 basic ear signals, one of which I have referred to above already.
When the cat is relaxed her ears naturally point forward and outward, in a waiting posture ready to be activated.
The ears jump to alert mode spontaneously and with great fluidity.
Pretty well any slight sound will result in a redirection of the ear
nearest the sound swiveling to face the sound to maximize the
reception. If she turns towards the source of the sound both ears are
pricked and pointed in that direction.
When she is agitated, the ears flick and twitch. If the cat has ear tufts such as possessed by some of the
grandest of the Maine Coons this ear twitching is made more noticeable.

When defensive we know that she will flatten her ears, unless of course you're a
Scottish Fold, who sends out the defensive signal all the time

.
The idea in flattening the
ears is the obvious one, to protect the them when the fight starts
("Battening down the hatches", in naval parlance).
Source:
Cat Watching - Desmond Morris
Me watching
Bottom photograph copyright Flicrk reproduced under creative commons