You do see that description places.

Often he does not look directly into his interlocutor's eyes, as though in order not burn him down.

My brother is like that, but I don't think there's any functional reason why every ISFj would do that. Hmm in fact my brother might do a bit of both - like look at you intently, and then look at the floor for a bit. I have no idea what most ISFjs do.