Hand rubbed Charas is thought to be the oldest form of cannabis concentrate, predating sieving by centuries, if not eons. It is made by rubbing the cannabis flowers so that the resin sticks to your hands, and then rolling it off our skin and collecting it in pressed balls. There were later versions using leather aprons and tales of vestal virgins running naked through the flowering cannabis plants, and having the resin scraped from their lithe young bodies.
Hand rubbing is an art form and in more recent times Charas is more likely to come from trimming scissors or from rubber gloves worn for handling and then placed in the freezer to make resin removal easier.
As you can see in the following picture, there is considerable plant debris to be picked off, and you not only can't get it all, but some skin cells will also be picked up, so Charas isn't sought for its purity, but for its flavor and effect.
Less skin from scissors and gloves, but plant debris continues to be an issue. Not necessarily a bad thing when you consider we smoke the plant material all by its lonesome and smack our lips in appreciation.
Hands from rubbing flowers so as to make Charas
I tried to find the article, but alas it evaded me, written by Jump117 on experiments he did rubbing flowers on glass panes, and then removing the resin deposited with a razor blade.
The best article I've seen on the subject, is Hashish by Robert Connell Clarke circa 1990 and available on Amazon.
Beads of Charas removed from hands
Charas beads formed into a ball.
Charas further worked into Temple Balls.