Passive pro refers to the climbing nuts, hexes, and tri cams on your rack. Although hexes and tri cams can "actively" cam into position when weighted, they are considered passive pro as they have no moving parts.
Whatever they might call them, almost every climbing manufacturer makes a set of nuts. They are all variations on essentially the same design – a tapered aluminum rectangle on one end slung with a factory installed steel wire. Climbing nuts are usually made from aluminum alloy, a material is both extremely strong and soft. It is important that the material have some give to ensure that the nut "bites" into the rock when loaded.
Modern machining techniques have transformed the original straight-sided nuts into curved and double-sided units. Very often climbing nuts are constructed with a transverse taper (when seen from the top, the piece narrows from face to face) to offer a wider range of placement options.
Hexcentrics or Hexes are six-sided semi-barrel shaped units with tapered edges similar in shape and curvature to modern nuts. Although well suited for wide cracks and bottlenecks, hexes were designed for parallel-sided cracks because of their ability to "cam" into the rock when loaded.
To some degree, mechanical camming devices have supplanted the popularity of hexes. However, a well-placed hex is one of the strongest anchors in existence. They have great range, they're light and easy to place, and hold well in icy cracks if you happen to be on an ice or alpine route. And if forced to retreat, they're much cheaper to leave behind than a cam.
Passive camming chocks, known as tri-cams, are a simple but effective design with no moving parts. They are good in parallel-sided vertical cracks because camming action wedges then into position when loaded. They work well in sandstone because the point digs into the soft rock. They can also be placed like a nut, particularly in pockets and holes, which adds placement options and versatility.
Read our article about Active Pro for more placement options.