The purpose of the Selection phase is to work out final revisions with the #1 Designer, and to select a winner once satisfied. Only the #1 ranked designer can submit logos in this phase. You can still move around ranks any time you wish.
When you have the logo you want, you can click the "Select Winner" button on your contest page. You will be asked to confirm your selection, and once confirmed there is no turning back. When the winner is selected you will no longer be able to request revisions.
Ensure your ranks 1, 2 and 3 are all different Designers. You must also have a total of 10 designs ranked.