Folks put a lot of stock in the pulse oximeter, as they should, because the pulse ox is an awesome feat of engineering and patient safety. But the pulse ox lags.
Here, the inestimable Dr. Jonas Pologe (rhymes with apology) demonstrates pulse ox lag with a breath hold.
1. When the sat is on its way down, the patient is more hypoxic than the pulse ox shows. This is another reason why, when laryngoscopy is not producing an acceptable view of the glottis, you should come out and reestablish ventilation/oxygenation earlier than you think. A more important reason to come out and bag early is described here.
2. When you are reestablishing oxygenation (using a bag mask, laryngeal mask, or endotracheal tube), do not use the pulse ox to judge the adequacy of ventilation, use capnography. That means the capnogram should be attached to the bag mask/LMA/ETT before the first breath is given. If the capnogram is good, ventilation is good, and the pulse ox will catch up, so relax and stop bagging so quickly.