A few brief comments about arm weight--
One of my teachers studied with Albion Metcalf who was a student of Tobias Matthay, who developed the concept of arm weight. So as you can image, I got a got a dose of that theory during my ten years studying with her.
First, think not of the just the fingers, but of the entire playing mechanism which includes the body trunk, shoulder, upper arm, elbow, forearm, wrist, hand and fingers. Each plays a vital role as the demands change within the music. For example, if you need to range over the compass of the keyboard playing heavy chords, the trunk and upper arm will play a predominant role in energizing and directing the entire playing mechanism. (In a situation like that, your legs and solidly planted feet also work to stabilize your entire body weight as it shifts from side to side.)
The elbow is important, for instance, in getting your hands to the right places on the keyboard and laterally choreographing the hands. If you doubt that, move your right elbow three inches outward, then look at the changed angle of your hand as compared to where it was when your elbow was closer to your body. I'm doing Scriabin's Etude, Op. 42, No. 6 in D flat. You cannot play that piece without the elbow of the RH making accommodations to facilitate hand positions. Similarly, the LH elbow has to "lead" the arm in arpeggios.
The forearm, wrist and fingers can play three entire different kinds of staccato touch, just to mention one touch, ranging from a heavier to a very light touch. The fingers are also going to take command during intricate passage work. The wrist will leave level position and vertically rise and fall occasionally as necessary to help choreograph minor hand position changes that cannot be accomplished by the elbow alone.
Now back to arm weight. Arm weight is not exactly a "free-fall" drop of the arm onto notes on the keyboard. Rather, it is a very relaxed yet controlled drop. You know when it's done correctly--despite the physicists and other naysayers who for years have claimed that you can play a piano well with a pencil eraser or an umbrella tip. In fact, you will attain a rich, singing tone that is unmistakable and undeniable to the accomplished pianist. The sensation of initiating the controlled drop is directed by and begins in the upper arm. In fff dynamic, even the trunk will participate, as you lean forward into the notes to increase power. There is another complexity that comes into play when arm weight is used in producing tone for chords. That is chord voicing. Arm weight and voicing must be coordinated and balanced to assure total clarity of playing at all times.
When is arm weight not a practical technique? When playing velocity. In that situation, arm weight becomes a hinderance rather than a help and will inherently work against speed. Arm weight, then, is used to best advantage within the lyrical side of the repertoire.
I hope this short explanation gives you some insights.
Topic: Arm Weight