首楞嚴義疏注經 2.64

2.64

下指，指下也。母陀羅，此云印。此意欲明見手不同有正有倒，以況其見亦有正倒。

'Pointing downward' means the fingers point down. Mudrā is translated here as 'seal.' The intention here is to clarify that seeing the hand differs in being upright or inverted, in order to illustrate that seeing also has upright and inverted.