Push gameplay is one of the many types of mapping techniques out there at the moment. The idea is to create an area that forces the player to move forward within the map and causes the area usually not able to be back tracked, preventing the player from reaching previous areas of the map. It is important to make sure that the player understands that it is not necessary to backtrack, otherwise they could possibly be frustrated by thinking they just can't figure out how to get back there.
One of the main ideas is to create an event that "pushes" the player forward. Mainly how you create that event is up to you but here are a few ways that mods and even Valve have implemented into their work.
One of the most common Push methods encountered is the Height Push. The Height Push forces the player to move or jump off a high point, and land a few feet below. This kind of Push is used most often because it is very easy to implement in a map. Once the player drops off the ledge then the player cannot go back due to the ledge being just a little too high to reach. One map in Half Life 2 Episode 1 utilizes this method; the player exits an old hospital and drops off a ledge that is about waist high. The player is unable to return to that area because the ledge is too high.
Besides ledges, here are some other ideas of how to enforce a Push using heights.
- Shafts (elevators, mines, etc)
- Ropes and ladders that end before they reach the floor
- A wind-tunnel could push a player up and deny any movement back downward.
Keep in mind that with the new physics-based puzzles that have been introduced since Half-Life 2, players may attempt to create their own path by stacking objects to climb. If you wish to avoid this, carefully plan the available props and/or weapons (such as a gravity gun) accordingly.
Another common Push method involves a disaster or destruction of the path back. In Half-Life 1, this was usually a tunnel caving in or a catwalk breaking. Other ideas include cliff-side paths that crumble beneath the player, outbreaks of fire or other hazards, and indefensible positions coming under heavy assault.
In Half-Life 2, when the player is cornered by the Metrocops directly before meeting Alyx, he may try to flee back up the stairs he just came down. If this is attempted, the stairs break. While this is mainly a height push, it obviously has a destruction element to it as well. Such combinations of methods do well, as it lessens the player's feeling of being a "rat in a maze".
A few other ideas that have been seen and used.