Just for your information, such usage is introduced in Cygwin user's guide.
It can be found under 1.6.3 File Access
The library exports several Cygwin-specific functions that can be used by external programs to convert a path or path list from
Win32 to POSIX or vice versa. Shell scripts and Makefiles cannot call these functions directly. Instead, they can do the same
path translations by executing the cygpath utility program that we provide with Cygwin.
and 3.1.9 Additional Path-related Information
The cygpath program provides the ability to translate between Win32 and POSIX pathnames in shell scripts. See cygpath(1) for
the details.