If I am following you correctly (and I'm not sure I am) what you are talking about is the "act of transfer". It is not a new rule but an old interpretation.
If the umpire rules that the player had control of the ball at the time of the play and that loss of control has come about from the act of transferring the ball from the glove to the throwing hand, the out can be recorded at the front end of a double play.....
example....ground ball to F6 who flips to F4....F4 catches the ball then while transferring the ball to his throwing hand drops the ball..... The umpire can call the runner out at second.......
In NFHS rules an Umpire can also call out the batter/runner if he felt that the slide was illegal and interfered with completing the double play.....
I do not believe that MLB has this last rule as taking out the fielder on a DP is a common and accepted practice in MLB games.
