If I've read this correctly, the theory states that depression heralds from damaged connections between synapses in a brain region regulating mood. Fixing these connections can be induced by stimulating the synapses in that area in a specific way, which Ketamine does. The effect appears to be transient, which means the damage returns after a while, requiring additional treatments. If there was a way to stop the disruption of connections in that brain region permanently, you would not need repeat Ketamine treatments.