Starbases that are taking massive amounts of damage have the ability to shut down all but the most important features of the control tower and surrounding structures to massively reinforce its shields.

What happens during reinforced mode?
The starbase force field becomes invulnerable to all damage but will no longer recharge itself to repair damage done to it.

The starbase control tower starts to burn fuel, strontium clathrates.
All structures requiring CPU will go offline and they can not be brought back online until the force field integrity is 50% or above again.

How does a starbase enter reinforced mode?
If the force field drops down to 25% shield integrity it will automatically enter reinforced mode if there are enough strontium clathrates in the control tower for at least 1 hour of reinforced mode.

What can we do during reinforced mode?
Structures that require no CPU to operate will still function. Additionally you can still target and remote repair defensive structures that are not inside the force field of the control tower and attempt to bring them online. Additionally, you may anchor structures at the control tower during reinforced mode.

What happens when the starbase comes out or reinforced mode?
The control tower will go online again. Any structures that went offline will have to be onlined again manually once the force field integrity is back at 50% or higher. The tower can be targetted normally and shields will start to recharge again.