ISAPI Rewrite allows setting rules that will be applied to URLs before they will be passed to IIS service. This test redirects accordingly to rule

RewriteRule ^(.*)/tests/isapi_rewrite_test\.redirect$ $1/tests/isapi_rewrite_test.html [I,O,R]

So, file /tests/isapi_rewrite_test.redirect on your site will be redirected to /tests/isapi_rewrite_test.html. If you see the message that claims that ISAPI Rewrite works, this means the rule were applied.