This sounds to me like just practising doesn't mean anything:
>>Commencing on the sixth calendar day prior to the club’s
seventh regular season game (including any bye week), and
continuing through the day after the club’s eleventh regular
season game (including any bye week), clubs are permitted to
begin practicing players on Reserve/Physically Unable to
Perform for a period not to exceed 21 days. At any time during
the 21-day practice period or not later than 4:00 p.m., New
York time, on the day after the conclusion of the 21-day
period, clubs are permitted to restore such players to their
Active/Inactive list, provided that no player may be activated
in a Week Six game. (Note: a club with a bye in the seventh
week will be treated as having a Sunday game)<<
EDIT:
But this does indeed seem you have to designate when player starts to practice:
2017-2018 National Football League Important Dates
EDIT 2: Reiss says he was wrong: