At the low pH the calcium sequestering machinery of the eggs is shut down and calcium ions are released into the cytoplasm. The pH rise is too quick for the calcium sequestering machinery to remove the excess calcium ions from the cytoplasm before they can activate the eggs. This is an easier, but different method. There are two ways to activate with ammonia. They give slightly different results as so it is a good idea to try both.
Ammonia is a weak base.
In its uncharged form NH4Cl it is able to pass the plasma membrane. Thereupon it re-equilibrates with in the inside of the egg. The pH outside of the egg is normally pH 8.0. The pH inside the egg is normally pH 7.0. The addition of the ammonia raises the internal pH of the egg to around 7.6 -7.8 or near the pH of the fertilized egg.