It is. Nginx launch a new worker while the other with the old config is still running, the master redirects new traffic to the worker and keeps the old worker until all previous requests has been handled. Once the old worker has finished the master process close it or when it time outs. It works even for updating nginx binaries.

It is a very useful approach and I use it all time as well.

I implemented the same in node by using the cluster api [1].

[1]: http://joseoncode.com/2015/01/18/reloading-node-with-no-down...