Yes Jean, I read it "somewhere."
“The guard protects the palm of the hand when holding the sword.”
The Samurai Sword – a Handbook, John M. Yumoto
"It prevents the hand from slipping onto the blade and brings the sword’s center of balance closer to the handle," Princeton University Art Museum
https://artmuseum.pr...ctions/objects/19430
"It protected its user’s hand when he grabbed his sword or prevented its user’s hand slipping towards the blade part when he hits his enemy." Samurai Museum Shop, Shinjuku
https://www.samuraim...81K19QIu6Ibei3h4WeIA
And there are more.
Anyway, I had no idea that this was a controversial topic, so I'll stop "spreading it" and get back to my original question: What function does the tsuba serve on a naginata?