Can someone explain how NIX_PATH works?

nix_path_confuses.txt

I understand how names are set explicitly on NIX_PATH:

$ NIX_PATH=nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos \
>     nix eval nixpkgs.lib.version 
"19.03pre155751.45a419ab5a2"

But this seems to indicate that raw paths can be put on NIX_PATH to override settings:

$ NIX_PATH=/home/tnks/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos \
>     nix eval nixpkgs.lib.version
"19.03pre155767.8070a6333f3"

However, this only seems to work when overriding, not in isolation:

$ NIX_PATH=/home/tnks/.nix-defexpr/channels \
>     nix eval nixpkgs.lib.version 
error: attribute 'nixpkgs' in selection path 'nixpkgs.lib.version' not found

What is going on here?