So if 5% of the miners don't upgrade, and produce a block (which will have nVersion<5), non-updated nodes will accept that block, but updated nodes will not. Once this happens, non-updated nodes will only accept new blocks from the 5% since the 95% will not be building off that No fork, but an increased number (~5%) of orphaned blocks.
Non-updated nodes will accept blocks version 5, because this is soft-fork