Use a prefix key to execute the key handler

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

exec/key-handler

@@ -1,9 +1,9 @@
 #!/bin/sh
 
 # Example for $XDG_CONFIG_HOME/sxiv/exec/key-handler
-# Called by sxiv(1) whenever an unbound key combo is used,
-# with the key combo as its first argument and the path of the current image
-# as its second argument.
+# Called by sxiv(1) when you press the prefix key (C-x by default) followed by
+# a key combo. The key combo is passed as its first argument and the path of
+# the current image as its second argument.
 # sxiv(1) blocks until this script terminates. It then checks if the image
 # has been modified and reloads it.