Braking - The braking (B) position of the running mode selector is unique to the Prius. Only use this when traveling down a long steep hill to save wear on the mechanical brakes or relieve an ache in your braking foot. If you engage this mode during normal driving you will at minimum ruin your fuel economy. If you don't live in a mountainous area, you may never need this mode.
Drive - The drive (D) position of the running mode selector is used in the same was as on any other automatic car. Since there is no conventional gearbox, the first (1) and second (2) gear settings are missing. You cannot "hold" a low gear to get additional acceleration since there is no low gear. If you feel you want to force a low gear to get additional engine braking, try the B setting.
ICE - Internal Combustion Engine, the gasoline-fueled engine that ultimately provides all the power used by the Prius. The Prius ICE is designed for economy and low emissions rather than power. When power is needed, the electric motor (MG2) helps out, using energy from the battery. Later, the battery is recharged using surplus power from the ICE or by regenerative braking. If the battery charge becomes too low, for example on a long steep hill, the Prius will make the best progress it can on ICE power alone.
MG1 - Motor/Generator 1, drives the sun gear of the power split device. To fully understand the use of this motor/generator, you need to understand the operation of the power split device. Simplistically, MG1 starts the ICE when needed and draws off some power to pass electrically to MG2 or to recharge the batteries.
MG2 - Motor/Generator 2, coupled to the ring gear of the power split device and hence the wheels via the final drive. This is the high-torque motor that drives the car, either along with or instead of the ICE, getting power from the battery and frequently also from MG1. Used as a generator, MG2 can also recover the kinetic energy of the car during braking and coasting to recharge the battery.
Neutral - As with any automatic car, the neutral (N) position of the running mode selector is not particularly interesting. You may use it if you want to push the car (but don't dingy tow the Prius!) but it is inadvisable, even illegal, to use this mode while at speed. Since the Prius has no conventional gear box, the effect of disengaging all gears is achieved by shutting off power to both motor/generators so that they spin free.
NHW20 - The second or third generation Prius, depending on your viewpoint, which is the one on sale today, model years 2004 to the present.
NHW11 - The first or second generation Prius, depending on your viewpoint, also known as the "classic" which was the first Prius offered for sale outside of Japan from model years 2001 to 2003.
NHW10 - The original Prius sold only in Japan from model years 1999 to 2000. Used NHW10s have shown up outside of Japan, particularly in Australia, in spite of the difficulty of getting repairs made.
Park - The park (P) position of the running mode selector is used in the same was as on any other automatic car. Since all other modes are set electronically, the mode selector could be a row of push buttons if not for park. In this mode, a mechanical linkage locks the transmission to prevent the car from moving. If you're used to a manual transmission and parking brake, watch for the car rolling a small distance before this mechanism engages.
Power Split Device (PSD) - This is Toyota's term for the epicyclic gear that connects the ICE, MG1 and MG2 .
Reverse - Put the running mode selector in this position (R) to go backwards. Bring the car to a stop first. Reverse on the Prius feels just like any other automatic car, except that the engine is likely to turn off. This is because the Prius reverses always and only under electric power. This obviates the need for a reverse gear in the transmission since the electronics can as easily run the motor backwards as forwards.
Ring Gear -
Sun Gear -
Turtle Mode -