To understand the need for SmartSync we need to refresh the basics.
When Open.HD first started development we wanted to easily change settings as many people working with this project and other branches are interested in experimenting with various frequencies, settings and configurations. Others were primarily interested in using it to just fly and enjoy the beautiful HD video. So, we wanted to make sure we kept things as simple as possible to maintain the plug & play ease-of-use that we want the project to have.
We accomplished a lot by using a dedicated settings and OSD application which makes it easy to change settings on the fly. This left us with two issues:
What is SmartSync?
SmartSync is a program that guarantees synchronization between any vehicle and the ground station that you have Open.HD installed on by making the air and ground always briefly start with fixed settings on a specific wifi channel (settable in QOpen.HD) then automatically matches whatever settings happened to be applied on the ground.
Be aware that the settings of the GROUND station are applied to the AIR vehicle. Make sure you save your profiles on the Ground Station and load the appropriate one before connecting to the AIR vehicle.
If you change any setting in QOpen.HD and save them with the AIR vehicle connected, they will be persisted automatically. SmartSync comes into play at boot, by default you don't have to worry about it.
If you ever have trouble, remember that GROUND has to be started before AIR and by default SmartSync is disabled!
If you want or need more control over SmartSync there are several options to prevent the default behavior. You can use GPIO 26 with a switch to disable SmartSync, or you can use a connected Joystick's Elevator channel during startup.

Aside from the methods mentioned to prevent SmartSync, we also have complete control over the system in the settings. Again, all settings are best modified through the QOpen.HD app.