Ｖｉｂｒａｔｏ

The rolling motion of the finger should be done very slowly at first,
flatting the main pitch - never sharping it -
otherwise it will give the aural impression of being the next half tone higher.

"Left-Hand technique" by Ruggiero Ricci, P55

下に向けてかけなければ上ずって聞こえる印象を与えてしまうって
>>8E40AhBIよりもずっとうまい（と思われる）リッチが言ってるぞ。