What is a Bluetooth "central"?
With wireless split keyboards, a "central" is needed to coordinate keys pressed on the two halves. If the two keyboard halves both connected directly to the user's computer (without a central to coordinate everything), it would not be possible to have basic functionality such as toggling a layer with a key on one half followed by pressing a key on the other half.
Do I need a dongle?
No, a dongle such as the Raytac USB Dongle is highly recommended but completely optional. There are benefits to having and not having a dongle.
What are the benefits of using a dongle?
The primary benefit of using a dongle is superior battery life. With a dongle, the keyboard halves only need to send wireless data (very little power required). The battery for the keyboard halves is expected to last 9 months with the included 400mAh battery.
If you plan on traveling frequently with your keyboard, you can still opt for a dongle setup. The recommended option is to keep your dongle plugged into a portable charger and using everything wirelessly.
What are the benefits of not using a dongle?
The primary benefit of not using a dongle is convenience. If you plan on traveling frequently with your keyboard, a dongleless setup is the most convenient as you won't need to plug anything into your laptop, tablet, or phone.
This comes at the cost of a much shorter battery life for the left central half as it needs to receive wireless data from the right peripheral half and that requires a lot of energy. With the included 400mAh battery, the left half is expected to last 3 weeks and the right peripheral half is expected to last 9 months.
How do keyboard halves communicate when using a dongle?
When the split keyboard is used with a dongle, the dongle serves as the "central" and the two keyboard halves are "peripherals". In such a setup, the two peripheral halves send keypresses to the dongle central and dongle is responsible for handling all the keymap logic.
The dongle can be plugged into one computer and acts like a standard USB keyboard. 5 additional devices (computers, tablets, phones, etc) can connect wireless to the dongle via Bluetooth.
How do keyboard halves communicate when not using a dongle?
Without a dongle, the left half is the "central" and the right half is a "peripheral". In such a setup, the right peripheral half sends keypresses to the left central half.
5 devices (computers, tablets, phones, etc) can connect wireless to the left central half via Bluetooth. The central half can also be plugged into a computer and used like a standard USB keyboard.
Is it possible to switch between using and not using a dongle?
Yes, you can switch between the two setups. However that requires resetting all the Bluetooth bonds and reflashing everything. The process can take few minutes so making the change frequently is not recommended.
It's currently not possible to dynamically select between using the dongle and the left half as the central. This is a firmware limitation and may change in the future though at the moment there are no concrete plans to implement such a feature.