Left thumb has to bend a little bit to hide under the palm when doing the "stand by" position (resting middle finger above 9th or 10th dot while the right hand is playing open string) and while left ring is doing Fan Yin, the harmonic sound.
泛聲若用名指, 掌藏大指. 有大指靠食指者. 單用大指時食指微曲覆蓋大指, 餘三指並直.
When left thumb is playing, left index has to bent a little bit and cover above the thumb, the rest three fingers keep straight and closed.
If there is combination of harmonics and open string sounds in one phrase, the right hand has to maintain a gentle playing.
指分遠近: (唐代陳拙論古琴指法-- 姚丙炎編著) 所以有清濁, 離岳近聲清硬, 離岳遠聲濯柔
Varies Right Hand Playing Positions (from Tang dynasty Chen-zhuo's Theory of the Guqin Fingerings edited by Mr. Yao Bing-yian 1921-1983). In order to create varies strong/soft, clear and sharper/ rounded and gentle sound qualities.
Position right hand between Yue mountain (the bridge) and the 1st dot, closer to the 1st dot -- Such as, Index finger plucking in or out, or Thumb plucking in or out, or Thumb and middle finger together doing Cuo.
Position right hand around the 1st dot -- Such as middle finger or ring finger plucking in or out.
Position right hand around the 3rd dot -- When right two fingers doing two continuing sound, such as Juan (index pluck in the 2nd string, middle pluck in the 3rd string for example), Yuán Lóu (index pluck in the 6th and middle pluck in the 4th for example) , or index finger pluck in two strings (2nd and 3rd string for example).
Position right hand around the 4th dot -- Such as Suǒ (Mǒ Tiāo Mǒ on one string), Shuāng Tán (middle finger out and index out on one string or two strings quickly), Lún (ring finger out, middle finger out and index out in sequence on one string), Lì (index out on two strings in sequence)
Position right hand in between the 5th and 6th dots -- Such as Fú (index plucking in several strings) and Gǔn (ring finger or middle finger plucking out several strings)
The more simple sound the more clear and bright. The more busy sound, the more soft and rounded. (Closer to the bridge, sound will be more clear and brighter; closer to 7th hui, sound will be more softer and rounded.)
The slower sound, heavier play. The faster sound, lightly play.