If the backend is excluded from load balancing, nghttpx will periodically attempt to make a connection to the backend. If the connection is successful N times in a row the backend is re-included in load balancing. If N is 0 a backend is never reconsidered for load balancing once it falls.
Please see https://nghttp2.org/documentation/nghttpx.1.html#cmdoption-nghttpx-b for more detail.