Multi-activation linedefs are linedefs that can activate different things each time they are crossed, so you can have a door that will only open the third time you pass near it, for example.
The method as explained step by step in Bug's Revolutionary Techniques for Doom and Doom II v2.3 guide:
- Trace a linedef which has the function lower sector with tag XXX to the lowest near.
- Create 2N - 1 adjacent sectors, where N is the number of times the linedef must be crossed before activation. I recommend to make them as thin as you can, put one after other, and use them to connect two rooms.
- Fix their floor height to be the same than the ceil.
- Give them the sector tag XXX.
- You might need to choose textures for the inner sectors.
- This technique is limited to lower and raise sector heights.
- If a player cross again the linedef too soon, it may be required more than N activations. The lapse of time is a function of the height of the N sectors.