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A team can put a player on waivers at any time.

Usually, that's a device that they use for sending a player who is out of options to the minor leagues (I'm not excactly sure how that works. I do know that with a young player, the parent team is allowed a certain number of "free" moves in which they can send him to the minors. But after a certain number - I'm gonna say three, but I'm not sure - the player must clear waivers first.

That means that any other team may claim the player (claims are honored in the inverse order of the standings, or W-L records), in which case the parent team has the option of either letting him go to the team that claims him for the waiver price ($20,000 I think), or withdrawing him from waivers and keeping him.

Generally, teams do not claim players on waivers just to block a trade. After all, they don't know when the shoe might be on the other foot. Plus, if they are out of the race and don't need that particular player, why claim him, especially if he has a big contract with time left on it.

There was a classic example, however, of a team claining a player to prevent another team from getting him.

I don't remember the year, but Atlanta and San Diego were locked in a duel for the NL West. Cincinnati (I think) put reliever Randy Myers on waivers, and San Diego claimed him to prevent Atlanta from getting him (Atlanta had no closer at the time, if I remember correctly).

He wound up not doing a thing for San Diego, and they were stuck with him and his big contract, which still had a couple of years left.

In these days of big salaries, waivers are used when non-contending teams wish to dump high salaried players.

They put a guy on waivers, and see who claims him. Then they know who's interested in the player, so they withdraw him from waivers and try to make a deal with the team that claimed him.

Anyway, in this case the Yankees got Lawton this way:

1- They discuss a deal with the Cubs. After the deal is agreed upon, the Cubs put Lawton on waivers. Or, the Cubs out Lawton on waivers, the yanks claimed him, the cubs saw that the Yanks were interested, so they withdre Lawton from waivers, and discussed a deal with the Yankees. Once the deal was agreed upon, they put Lawton back on waivers, and the Yanks claimed him again.
2- No team with a worse W-L record than the Yankees claims him, mostly because the teams with a worse record are already out of it, and they would have to pay Lawton the remaining $1 million on his contract for this season.
3- The Yankees claim him, and the Cubs do not withdraw him from waivers, because the Yanks have agreed with the Cubs that they would send the Cubs the two minor leaguers that they agreed upon, who do not have to clear waivers since they are not on the Yankees 40 man roster.

Also, to be eligible for the post season, a player has to be on the 40 man roster by August 31st.