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 and artists until voting begins. 'Hidden' designs will of course not have the chance to receive any critique from others and they will not appear in the Leaderboard.
When you are logged in, you can access your own 'hidden' designs in the challenge gallery, in the 'hidden designs' section of your designer profile, and in the 'Manage Your Collection' section of your designer profile.
If you'd like to "unhide" a design, you can do so by following the instructions here: How do I "unhide" my submission?