For fullest tone, both in singing passages and everywhere else, you need a still more substantial basis than the fullest arm-weight can supply, unaided. Here you must learn to add a down-exertion of the forearm to the previously discussed momentary weight-release of the whole arm. That is, you must leave free the weight of the whole arm, while you nevertheless exert your Forearm downwards; that is*, a down-exertion of the Forearm in addition to this whole-arm Weight.

Note: This conjoint Forearm down-exertion with released Weight is needed for a certain penetrating singing-tone. Pure Weight-release does not suffice here. Use of the This exertion of the Forearm downwards is again quite a natural Forearm lever action is really a lifting action, for it is similar to the natural use of the bannisters^[1] as a help in mounting the stairs. The shoulder, therefore, in this case becomes the Basis for your playing action it is lifted against, and thus, through your shoulder, the whole mass of your body (quietly resting on the chair) becomes the ultimate Basis.
The main secret of producing large tone without harshness lies in this down-exertion of the forearm in conjunction with a free upper-arm.

Finally, we see that a fourth optional form of arm-use is possible. It is one, however, that should be carefully avoided unless used very lightly.
Here, along with the down exertion of the forearm, you drive forwards with the upperarm, instead of leaving it lax. It feels like a horrible digging action into the keyboard with arm and elbow, and the elbow feels thoroughly rigid during its perpetration. This type of motion is to be avoided.
This last (and undesirable) form of arm-condition calls for more amplified consideration, as it forms one of the worst and most frequent causes of bad tone, and perhaps more often spoils musicality of effect than any of the many other possible forms of incorrect motions at the keyboard. It is only too frequently heard in the playing of otherwise quite good pianists, and even popular artists. Only keyboardists with sensitive hearing and an understanding of the playing mechanism avoid it consistently.

This forward action of the upperarm can actually help us to obtain the very lightest effects. But in order to succeed in motion, we must carefully avoid any evidence of fore-arm down-drive with it, and must also eliminate almost all of the forearm weight. Thus, when you lightly the upper-arm forward (away from you) you really lift its weight off the keyboard visibly or invisibly, as the case may be, and you can thus achieve lightness to any degree. But do not overdo this trick, or else you will miss your notes.

[1]bannister: The structure formed by the uprights and handrail at the side of a staircase.