There is no better demonstration of the effectiveness of the parallel set exercises (chord attack) than using them to learn the trill. There are only two problems to solve in order to trill: (1) speed (with control) and (2) to continue it as long as desired. The parallel set exercises were designed to solve exactly these types of problems and therefore work very well for practicing trills. Whiteside describes a method for practicing the trill which is a type of chord attack. Thus use of the chord attack for practicing the trill is nothing new. However, because we now understand the learning mechanism in more detail, we can design the most direct and effective approach by using parallel sets.
The first problem to solve is the initial two notes. If the first two notes are not started properly, learning the trill becomes a difficult task. The importance of the first two notes applies to runs, arpeggios, etc., also. But the solution is almost trivial -- apply the two note parallel set exercise. Therefore, for a 2323.... trill, use the first 3 as the conjunction and get the first two notes right. Then practice the 32, then 232, etc. It's that simple! Try it! It works like magic!
The trill consists of 2 motions: a finger motion and forearm rotation. Therefore, practice the 2 skills separately. First use only the fingers to trill, with the hand and arm completely still. Then keep the fingers fixed and trill only with arm rotation. This way, you will find out if it is the fingers or arm rotation that is slowing you down. Many students have never practiced rapid arm rotation (arm rocking), and this will often be the slower motion. For fast trills, this back-and-forth rotation is invisibly small, but necessary. Apply the parallel set exercises to both the finger and arm rotation motions. Exaggerate the motions for slow trills and increase the speed by reducing the magnitude of the motions. The final speed of both motions need not be the same because you will use a smaller motion for the slower one in order to compensate for its slowness. As you practice these, experiment with different finger positions. See the Tremolo section for a similar situation – the trill is just a shrunken tremolo.
Relaxation is even more critical for the trill than almost any other technique because of the need for rapid momentum balance; that is, the parallel sets, being only two notes, are too short for us to rely solely on parallelism to attain speed. Thus we must be able to change the momenta of the fingers rapidly. For trills, the momentum of the finger motion must be counteracted by the arm rotation. Stress will lock the fingers to the larger members such as palms and hands thus increasing the effective mass of the fingers. Larger mass means slower motion: witness the fact that the hummingbird can flap its wings faster than the condor and small insects even faster than the hummingbird. This is true even if the air resistance were ignored; in fact, the air is effectively more viscous to the hummingbird than to the condor and for a small insect, the air is almost as viscous as water is to a big fish; yet insects can flap their wings rapidly because the wing mass is so small. It is therefore important to incorporate complete relaxation into the trill from the very beginning, thus freeing the fingers from the hand. Trill is one skill that requires constant maintenance. If you want to be a good triller, you will need to practice trilling every day. The chord attack is the best procedure for keeping the trill in top shape, especially if you had not used it for a while and feel like you might be losing it, or if you want to continue improving it.
Finally, the trill is not a series of staccatos. The fingertips must be at the bottom of the keydrop as long as possible; i.e., the backchecks must be engaged for every note. Take careful note of the minimum lift necessary for the repetition to work. Those who practice on grands should be aware that this lift distance can be almost twice as high for an upright. Faster trills require smaller lifts; therefore, on an upright, you may have to slow down the trill.