From Quake Wiki
This is the brush entity used to make the wind tunnels that push players around. Simply make a brush to cover the wind's area of influence (the area in which the player gets pushed if he/she touches it), then convert it to the trigger_push entity. Set the "speed" keyvalue to the amount of push strength you want the entity to have, and the angle to the direction the player is to be pushed (Note: You can't set this value to 0. If you want to push in that direction, use 360). To see a sample map with the trigger_push entity in use, download this map. Note the room with several trigger_push entities near each other. You can use this room in Quake to see a comparison of push strengths by how high you're pushed (use the walls to judge how high you are being pushed). Notice that there is a limit on how effective the push can be. Any push strength higher than about 300 won't have any increased effect.
The only things affected by trigger_push are players, monsters, and grenades. For monsters however, there is a caveat: the monster must already be airborne by other means, for the trigger_push to have any effect. This means you will need to propel your monster into the trigger_push volume by using a separate trigger_monsterjump.