The waivers thing came from the Ryan O’Reilly situation a few years back. While he was an RFA for Colorado and holding out for a new contract, he had played a couple of games in the KHL then the Flames tendered him an offer sheet.
I think the rule was that anyone from the KHL that signs with a club during the NHL season has to clear waivers on his way back to the NHL. A couple of players (like Nabakov), who signed with one team, had been picked up by different teams through this mechanism.
Some people thought that because O’Reilly wasn’t the Flames property, that not only would they have to give up draft picks for signing an RFA, but could lose him under those waiver rules. No one really knows for sure if they would have lost him (the Flames had said that they would have fought this rule as it wasn’t their interpretation) as Colorado matched the offer and he didn’t go through waivers as he was Colorado’s RFA.
Then in the 2013 CBA, they made this amendment / clarification:
According to the 2013 CBA summary document, if the player is already signed by an NHL team when he goes to Europe, then he has to clear waivers when he returns. But if he’s unsigned (e.g. Ryan O’Reilly), then he doesn’t have to clear waivers. Such a player can sign with his team mid-season and return from Europe without having to go through waivers first.