diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b50711c..534ef44 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5626,6 +5626,10 @@
 	Note that when using the NFA engine and the pattern contains something
 	that is not supported the pattern will not match.  This is only useful
 	for debugging the regexp engine.
+	Using automatic selection enables Vim to switch the engine, if the
+	default engine becomes too costly.  E.g., when the NFA engine uses too
+	many states.  This should prevent Vim from hanging on a combination of
+	a complex pattern with long text.
 
 		*'relativenumber'* *'rnu'* *'norelativenumber'* *'nornu'*
 'relativenumber' 'rnu'	boolean	(default off)