How the dumb proxies/blind relay hanppen in detail?

I have some confusion about the dumb proxies/blind relay.

The http head's keep-alive will keep the tcp connect,so i think that the client and server both not to send the FIN.

Reference 《HTTP:The Definitive Guide》,page 95

"So the proxy will hang waiting for the connection to close"

"Because the proxy never expects another request on the same connection,the request is ignored and the brower just spins ,making no progress."

Why this situation happened?

If client does not send FIN ,why the proxy will ignore the request by client? If client send FIN ,how the TCP connection alive?

Thanks!