The answer to this question says:
The opening thread defines where is the best place to slash the sword that will behead them in a single swing. The opening thread scene only happens during Tanjiro's fight as it represents the keen sense of smell he possesses. This constantly reminds us how Tanjiro is different from others as others learn to do that through experience and training but he can do it just by sensing it.
From the answer, "Opening thread" determines the angle and position of sword to be needed to successfully kill a demon and experienced demon slayer just learns from experience but Tanjiro has a olfactory ability to sense the "Opening Thread".
This was evident during the battle between Rui and Tanjiro. Tanjiro was struggling to get hold of the "opening thread" and although he managed to behead Rui using Hinokami no Kagura, Rui was surprisingly not dead and he managed to sew himself. Later, Tomioka came and beheaded Rui with no effort whatsoever. So, do experienced demon slayers looks for "Opening thread" or they just go by intuition? In other words, is Tanjiro the only one who can sense a "Opening thread"?
And how does newcomers determine the perfect slash to behead the head of demons? Is it from their individual ability like Inosuke throwing a full blown attack while Zenitsu becoming precise in sleep mode?