The value of the sexp you passed as its second arg is the symbol delete-trailing-whitespace,
because the arg to when is a non-nil list (because you quoted it). So the when condition is always true.
And you never invoke function delete-trailing-whitespace; you just return the symbol that names that function.
The diminish mode is activated, which helps us to keep the modeline clean.
As we see, in the modeline the modes that are activated are shown, and every time
we have more activated minor-modes, which add small functionalities to Emacs.
In this case, there are many modes, that if they do not appear, nothing happens.
So we hide some, like undo-tree that will always be activated.
Active auto-revert-mode that automatically reloads modified files out of Emacs.
It is very useful to see logs (like auto-revert-tail-mode) among many other cases.
