If you decide not to sign your rookies. It will automatically become UFA and available for anyone to bid EXCEPT YOU! (If you bid on that player, it will automatically goes to exactly same salary as you drafted him).
You can't do this way:
Ex.
Drafted Player A 1st round and 1st pick worth 1.5m and didn't sign him... then someone else bids him for 750k, you bid 800k. If you want him back you MUST bid at least EXACTLY same amount as you drafted him or more.
Thanks