diff --git a/README.md b/README.md
index f4dc509..61c3292 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,20 @@ The main goal is to launch applications directly into the same directory
 as the focused applications. This is especially useful if you want to open
 a new terminal for debugging or compiling purpose.
 
+How does it works
+-----------------
+Since there is no proper options to get the pid of the currently focused
+windows,
+xcwd first try to read the \_NET\_WM\_PID property. If it fails, it reads the
+\_NET\_WM\_CLASS and compares it to the name of all the processes (it's kind of
+`pidof name`).
+
+When xcwd has got the PID, it search the deepest child he has, thus avoiding
+getting the working directory of the terminal emulator instead of the shell.
+
+Finally it prints the content of `/proc/pid/cwd` on the standard output.  If
+xcwd was unable to find the PID, it prints the content of the HOME variable.
+
 Requirements
 ------------
 - libX11-dev