Sync is restart your production to send all units together. The most of users sync only the same unit because they knows there's a different time to each unit. Example: Starting the defender and the sniper at the same time, they won't spawn together. To restart your unit, you must select the building and hit esc key or hit the cancel button. 
-A single monkey ... is weak. -Monkeys together - a force
Sync is important to battle, when you sync you'll be fighting as group. As you can see the naga will die easily.
To sync you need to know the exactly time of spawn. Old players are used to play with overall cost. The cost is the mechanism. A good example: chaos race The overall to Demon Quill is 350 ( 120 + 230 ) The overall to Chaos Champion is 350 ( 160 + 190 ) The Red Dragon cost is 350 These units has the same final cost, then you can send them at the same time, they will spawn together. But and the Infernal (500)? We could start the infernal first and wait for x time and then start to send the rest of them? Yes, it would work however if you have the exactly time you don't have to fear the game.
You have to make the math [overall cost / 20 +15] = time build
Archer costs 180 then? 180 / 20 = 9 9 + 15 = 24 Each 24 seconds will spawn a new Archer.
Master Archer costs 200, the archer 180, then.. 180 + 200 = 380 380 / 20 = 19 19 + 15 = 34 Each 34 seconds will spawn a new Master Archer
You are not a big fan of mathematic? no problem. Click in your building to see the time to send an unit.
So if you have Master Archer and Archer, start the M. Archer and after 10 seconds you start the Archer. They will spawn together.
Stacking, please read the article Where to build.
Send the next longest unit. Make the same to other units and you will have a good result. A good formation, the melee, ranged unit, casters, support, siege and air. Yes, you can however you can sync with an unit. And if this unit require less time to send than yours, then you only can sync once. Example. You have Naga Royal Guardian, 37 seconds The bot/pub got Master Archer, 34 seconds. Set the -mt to 34. When left 3 seconds to send another M. Archer, you start the Naga. | 
