The Old Way...
Wanda Landowska (1879-1959)
Over time, the demands for a certain technical skill on the piano (or its predecessors) have changed.
This is partly because the instrument itself has gone through so many transformations. From the lighter played Hammer Clavier, to the heavier hammer and key action of today's Grand Pianos, you need a different piano technique to play well.
But not too long ago, piano teachers insisted on "curve-your-fingers-or-else...” with exaggerated curved fingers and resulting tension and pain, which was a technique derived from earlier instruments and styles of playing.
As well as to "helpfully" push the elbows of a student outwards, insisting they would be kept that way, with excruciating neck and back pain as a result. (To look nice?)
Maybe the old saying "No pain, no gain" had something to do with it...?
Technical advice of earlier times reflects its own time, and might not always be useful for the instruments we play today.
Because of the ever increasing demand of improving piano technique, skills and musical excellence (due to the enormous worldwide competition pianists face today), some of the advice from earlier than the 20th century might even be causing injury.
That aside- there's a lot we can learn from earlier technical advice (before 1800s) on how to play keyboard instruments. For example it can be used at advanced levels of interpretation to give us a better understanding of how earlier styles of music were expected to be played.
With the development of the modern instrument (we still play today), with a heavier key action but also a greater sensitivity to the color of sound, the demand for a new technique caused pianists and pedagogues such as Tobias Matthay in the early 1900’s, to write about piano touch and balance, or "weight effect". Get a well-used copy here for free.