The Shulchan Aruch OC (100:1) writes that there is an obligation to learn the tefillos which are not commonly said beforehand, and the Rema notes that nowadays we don't need to review the prayers because we have a Siddur. However, the Mishna Berura (100:3) adds:
מתוך הסידור - ותפילות ופיוטים שחמור פירושם צריך להסדיר תחלה ולא מהני ספר בזה - The tefillos and piyutim are more stringent and must be reviewed beforehand since a Siddur doesn't help with this.
Seemingly, the Chofetz Chaim would require us to review the entire Rosh Hashana/Yom Kippur Machzor before the Yomim Noraim. Is this something everyone does that I'm just not aware of or is there a heter we rely on today not to spend a while reviewing those tefillos beforehand?