What was the mysterious ingredient in making of Japanese sweet bean paste (an or あん) from the movie?
The ingredient is Mizuame (水飴), a Japanese sweetener. It adds sweetness and gives a luster.

When they need large amount, professional cooks usually scoops it with their bare hands, as she does in the movie.  I'm not sure of the reason, but Mizuame is very sticky and it is troublesome to handle with tools.

And this behavior explains that she has long experiences in making read bean paste.