So basically, the plank is a way to enforce a minimum rideheight
Yes, the plank was created for that reason. A fully flat-bottomed car was required for all F1 cars starting in 1983, since sliding (and fixed) skirts were eliminated at the end of 1982, to slow down cornering speeds, and fully enforce the 6cm ride-height rule. Teams were experimenting with cars that lowered as they left the pits, especially just after scrutineering.
The plank came about in mid-1994, which raised the height of an F1 car's side pods and wings to 10cm. Again, this was to reduce cornering speeds. Cars still have to flat along the plank's sides and the plank as to be fully flat. There's also a limit to how much of the plank can "naturally" wear away during a race (I forget the amount), to prevent teams from creating the fuly-flat bottom and getting back a lower ride height.
Benetton got caught doing this at the 1994 Belgian GP, and Michael Schumacher was disqualified from his victory. There was a dispute over this, since he'd spun midway during the race, and ran over a long kerb, which the team argued had abraded the plank, which was the cause of the too-thin plank. I think it remains the last time in F1 a driver lost his win due to disqualification.
...a flexible floor simply moves the other parts of the floor lower, achieving the same effect as an overall lower car?
Exactly, and a ban on movable aerodynamic devices has been in place for many years now, which extendes to flexing front wings of the Ferrari (a slight bit of "deflection" is allowed, to prevent shattering due to normal use), the mass damper of last year's Renault R26, the never-fully-raced Lotus 81 (the whole car moved as two chassis), and the attempt by Ferrari to create a rear wing that could be modified by the driver...in 1968!
):
. The 81 raced, but the 88 was banned.
