Modern piano students often see a trill and begin on the written principal note before alternating rapidly with the note above. In Bach, a trill commonly begins on the upper auxiliary. The ornament table Bach prepared for his son Wilhelm Friedemann demonstrates the basic trill as an alternation beginning above the principal note. This was more than an arbitrary period convention.
The upper note often enters on the beat as a brief dissonance against the harmony, then resolves to the principal note. A trill is therefore not merely a vibration around one pitch. It compresses expectation and resolution into a short span. Beginning directly on the principal note may sound smoother, but it can weaken that directed movement. The first note should be clearly placed on the beat without becoming a hard accent; the fingers should follow the harmonic tension rather than chase speed alone.
Even so, “Bach trills always begin from above” is too crude a rule. Tempo, note value, the preceding pitch, voice leading and the particular ornament sign can all affect the execution. Bach’s table supplies a grammar, not an identical fingering formula for every passage. What matters is hearing how the ornament gives the principal note direction and weight.
https://imslp.org/wiki/Explication_unterschiedlicher_Zeichen_(Bach%2C_Johann_Sebastian)
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