The basics

Every unit spawning building has a set mana value under its hp which signifies how long it takes for this building to spawn a new unit. It doesn't function as mana, it is only informative. The "synchronisation spell" makes use of these values to connect a building with lower or equal spawn times to another building, which allows both of them to spawn their units at the exact same time. The progress towards spawning a new unit is signified by a small bar above it. It is important to take note of these as synchronisation will reset the progress bar of the connecting building (child), if the building it connects to (parent) spawns its unit.

The benefits

• Units arrive in a controlled formation, squishy units at the back and tanks at the front.
• Groups of units have a much bigger impact when arriving to the battle, than lone ones.
• It is less likely that single units will get picked off easily, as the grouped units will protect each other.

The issues

• Syncing will reset spawn time of the child buildings. Do it at the wrong time and it can cost you your castle.
• Syncing quicker buildings to slow ones essentially lowers their spawn speed = you get a smaller army overall.
• Using very slow buildings as parents for sync (ex. legendary buildings) can absolutely lose you the game.
• Every time you use the "sync spell" the spawn point of units changes.
• A group of units will get hurt more by enemy buildings with aoe dmg spells.

Manual synchronisation

It is considered an advanced technique which involves synchronising buildings without the use of the "sync spell". You turn off unit production until a right moment, then you turn it back on. This not only means that you do not lose a spawn of units at the wrong moment but also allows you to precisely control when the units spawn (and by an extend, where it will sit within the formation).