You likely remember when the Mets picked up Steven Register from the Colorado Rockies last season. He almost made the team, but was eventually shipped back to the Rox with $50,000 payment.
Here's a primer on the Rule V draft:
Players who are not currently on their team's 40-man roster are eligible to be selected in the Rule 5 draft, but only after a standard exemption period has elapsed.Make sense? It's not too hard. Pretty much if you draft a player, he must be on your MLB team and active for at least 90 days, or he must go back to the team from whence he came.If chosen in the Rule 5 draft, a player must be kept on the selecting team's 25-man major league roster for the entire season after the draft--he may not be optioned or designated to the minors. The selecting team may, at any time, waive the Rule 5 draftee. If a Rule 5 draftee clears waivers by not signing with a new MLB team, he must be offered back to the original team, effectively canceling the Rule 5 draft choice. Once a Rule 5 draftee spends an entire season on his new team's 25-man roster, his status reverts to normal and he may be optioned or designated for assignment.
To prevent the abuse of the Rule 5 draft, the rule also states that the draftee must be active for at least 90 days. This keeps teams from drafting players, then placing them on the disabled list for the majority of the season. For example, if a Rule 5 draftee was only active for 67 days in his first season with his new club, he must be active for an additional 23 games in his second season to satisfy the Rule 5 requirements.
Any player chosen in the Rule 5 draft may be traded to any team while under the Rule 5 restrictions, but the restrictions transfer to the new team. If the new team does not want to keep the player on its 25-man roster for the season, he must be offered back to the team of which he was a member when chosen in the draft.
There are a few rules and exemptions, so check out the Wikipedia article for the full story.