This goes for internal users such as yourself as well, when it’s enabled this means it will force your browser to always see the same variation.
This behaviour carries over until you clear the browsing data (cookies & session data) which can make testing a bit of a headache.
The best way to test this is to open the test page in an Incognito/Private Browsing window. This gives you a completely fresh session without any browsing data.
In this case Simple Page Tester will pick the variation randomly on the first visit, then if you want to test again, you need to close the Incognito window altogether and open up a fresh one.
If you don’t close the Incognito browser windows altogether, it will treat it as the same session and you’ll always see the same variation.
When doing testing of your redirects in this manner please keep in mind that the variations are chosen at random and therefore you may need to repeat the test a few times in order to see the other variation in the test.