Equalising with the base of your tongue instead of your lungs: you drive the small pocket of air in your mouth towards the middle ear. Reliable to 25–30 metres, and beyond with training.
With Valsalva you push air out of your lungs into your ears using abdomen and chest. That works fine near the surface, but under pressure your lungs keep shrinking, and eventually there is simply no air left to push. Frenzel sidesteps the problem: with your glottis closed, the base of your tongue drives that small pocket of air upwards, decoupled from your lung volume. The side effect is what decides things in freediving — chest and belly stay relaxed, saving the oxygen you need at depth. The most common mistake is still not technical but a matter of timing: most people equalise only once they feel pressure, and by then gentle equalising is off the table. Start at the surface and repeat every one to two metres. Never push against pain — if it will not go, ascend a little and try again. With a blocked nose nothing works at all, and that dive is simply off.