Some performers at the organ slip into the habit of adopting certain mannerisms (unnecessary movements) as they play.
This is, in some cases, a carry-over from enthusiastic playing of the piano, an instrument which changes its volume and tone depending upon how hard the keys are struck.
In organ playing however, there's an old dictum which says "all unnecessary movement is harmful because it's a waste of time and effort;" this holds true with and can be applied to a wide variety of applications in life, not just organ playing (photo).
Organ playing requires no physical strain, nor does it require a litany of mannerisms to communicate a language of warmth and meaning to an audience.
When we see performers swaying on the bench as they play, shaking their heads as if they were trying to paint a fence with a brush between their teeth, arching their back and lifting their elbows on every strong beat, rhythmically bobbing their head as they crunch their entire body over the keys then quickly straightening their spine, jerking their head left and right and their jaw up and down with every note of the subject and every beat between entries, raising one hand high at the end of each phrase, rapidly flipping their hands upward when releasing final chords as if they just touched a hot stove, sometimes even throwing their hands three feet into the air when the piece concludes, rotating their elbows rhythmically like they're trying to do the breast stroke, giggling their head and grimacing as if they were either in a trance, in the throes of an epileptic fit, or experiencing the end of a spell of constipation, etc., it's more fun than a movie; it's important to realize however that NONE of these visual mannerisms have ANY effect whatsoever on tone production; some players cultivate them simply to draw off some the energy they feel as the music unfolds, but they certainly must know that, in the process, they're putting on a show [See blog, Bach d minor, Part III].
These kinds of antics tend to make onlookers think that the organist is actually doing something; they're also fully aware that if they're consistent with a certain mannerism, it tends to set them apart from the general crowd of players and give them their own signature identity.
As long as nothing foreign and destructive to the composer's music is inserted which takes a wrecking ball to it, such as by interrupting the majestic counterpoint at the final cadence of a Bach fugue with a home-made cadenza which efficiently annihilates what Bach intended with a meaningless commentary, all of these "harmless" mannerisms aren't the worst that can happen in organ playing.
Bottom line: The grandmother listening in the back row and the little boy listening in the front row who can't see the organist upstairs don't give a flying flip if the organist is sitting up straight, bobbing the head, making faces, chewing gum, rolling the eyes, slobbering at the mouth, wearing a dew rag, releasing final big chords with the right hand drawn quickly back with a turned wrist, or swaying on the bench like a drunken sailor; all they expect is to hear some organ music, to hear it clearly, to be moved by it, and, in case they've heard it before, to hear it the same way without needless tampering [See blog, Touch, Part V].
Mannerisms in organ playing impress only those who are listening with their eyes.