Hello, can someone tell me what it means to choke a transformer and the theory behind it. When would you do this and why?

The OCP requirements give the maximum rating that can be used to protect a transformer.

In the case of a recent design here, the load was just beyond the capacity of a 300KVA transformer, but the next size is 500KVA.
In order to avoid the costs and physical size of the OCP on both sides of the transformer, it was fused at something less that the Code maximum. Since that will also limit the secondary side current, the smaller distribution equipment can be used on that side as well. I rarely choke a transformer, but sometimes the big jump in sizes for the big ones get to be problems. In this case, I would have needed a 350KVA transformer (if anybody sold one), but I was able to choke a 500KVA without having to use the maximum OCP sizes in the design.