Translation
Thereupon the palm of the hand was sent from Him and the writing was inscribed.
Paraphrase
Therefore the disappearing, flat part of a hand which disperses things (in this case truth) was sent from the Lord of the heavens and truth was communicated permanently and irrevocably.
Why Just the Palm of the Hand?
If this use of the “palm of the hand” seems strange to you, that is because it is not about what physical part of the hand was being seen; it was about the root word from which the word “palm” came. Its origin was in a word meaning first of all “to disappear or disperse,” then by implication “something flat which could disperse things,” e.g. the flat part of the hand, i.e. the palm of the hand. The question is not “What did he see?” but rather “What did it mean?”