7. The area of contact with the strings is large. If you press your fingers onto the strings in playing position, the resulting indentations in your finger go right across the middle of the finger end (the same applies to the thumb – you are playing with the middle of it, not the edge). You are not scraping away on the top of the strings, but getting right in there with them!
8. It is important to play both strings of a course cleanly, equally, and simultaneously. Bass courses tuned in octaves make this easy to check – you should hear the two notes sound absolutely together.
9. In plucking the fingers displace the string slightly towards the soundboard, so the plane in which the strings vibrate is not parallel to the soundboard (which would give very little sound) but angled. In some ways this is analogous to the apoyando stroke of the guitarist.
10. A trap which is easy to fall into is “bouncing”: when playing runs with the thumb and first finger, moving the hand towards the soundboard and away from it with each stroke; or when playing chords, pulling the fingers away from the strings as the chord is plucked. Don’t do it! The knuckles should stay the same distance away from the soundboard all the time.
11. The middle finger is more difficult for some people than others. It’s difficult for me because, unlike most people, I have a long 3rd finger (only 6mm shorter than the middle finger) and a short index finger (about 12mm shorter than the middle finger). The tendency is to bend the middle finger, but we have said the last joint of all the fingers should be straight (and relaxed). For me, that means the middle finger plucks about 12-14mm further from the bridge than the index. In fact the index finger comes roughly midway between the middle finger and the thumb (well, actually slightly nearer the middle finger, my thumb is quite short). Oh, I’ve just noticed another interesting fact: in spite of its length, the ring finger plucks in exactly the same place as the index (it starts with more bend in the second joint), so reading from the bridge, you get thumb, then index and ring (and little finger on the soundboard, all at the same distance from the bridge), then middle finger. The angle between the middle finger and the strings is therefore slightly shallower than it is for the other two fingers.
The really crucial point is that people’s hands do vary considerably, therefore different players look different even when their technique is very similar. The principles underlying the technique will be the same.