Right before you submit your design to a challenge, you are given the option to "hide" your design during the critique phase. If you opt to do so, then you will able to submit your design, but it will be invisible to all other designers until voting begins. You can see all those designs in the 'hidden designs" section of your profile when you're logged in. Other designers cannot see your 'hidden designs' section. 'Hidden' designs will of course not have the chance to receive any critique from others and they will not appear in the Leaderboard. If you start a poll with your hidden design, it will show-up under, "polling now" on the main challenge page.
If you'd like to "unhide" a design, you can do so by following the instructions here: How do I "unhide" my submission?