As you go deeper, the air in your lungs shrinks under pressure until there is too little left to draw on for equalizing. The mouthfill technique solves this by capturing a volume of air in your mouth at a chosen depth and then using only that stored air to keep equalizing as you continue down. It is the gateway skill for serious depth.
Build the foundation first
Mouthfill is not a beginner shortcut. You need a dependable, automatic Frenzel and clear control of your soft palate and glottis before this technique will make sense or be safe.