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Fully on, current is flowing (>10 mA), dioes looks like a small resistance (1-3 ohms)
Fully off, a reverse bias voltage of more than 5 volts is applied to the diode. Almost no dc current flows. Diode looks like a small capacitance (1 to 0.1 pF) with a large RF resistance (3 K or so) in parallel
In between, diode has a forwavd current flowing, but it is too small to fully turn on the diode, maybe in the 100 uA to 1 mA range. Diode looks like a resistor (10 to 100 ohms) with a large capacitance (maybe 10 to 100 times the reverse bias capacitance). You would use the diode in this mode if you were trying to make a variable RF attenuator.
DC Modes:
When you are trying to turn the diode from off to on, you need to inject electrons into the I region as quickly as possible. You sometimes need to place a momentary spike of voltage across the diode in order to inject that charge quicker.
When turning the diode from on to off, you need to deplete all the charge that is stored in the diode's I region. It is like discharging a 0.1 uF capacitory quickly (a surprise, because we are talking about a diode that might look like an RF capacitance of < 1pF).
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