That is correct...in to ford...out the rest of the time. The bellhousing vents the oil from the leaky rear main seal on the 230, among other reasons for it being there...in the fording application, the fording valve pressurizes the axles, gearboxes and master cylinder. With the plug out, the pressurization wouldnt work and would leak out this hole. With it in, one will notice the boot on the shifter will actually inflate, thus telling you the fording system is operating as designed.
The reason for the pressure is that when one is going through water there is pressure from inside the axle or gearbox going outward, so if there is a leak it will be from the inside out pushing water away, instead of driving in water and possibly having it leak in to those components.
If you dont have the fording kit with he fording valve to pressurize the stuff, it still keeps water and mud and etc off the clutch...a good thing to protect.