A nice technical gimmick.

If you run up the stairs from Endrods, you can fall through between the second and third step and if you then use a Redstone flare or Redstonedust on the Sync by honeybunny inconspicuous diablock (You can of course also exchange it :P), it will be destroyed first, then a secret door will open, which will then close again.

Instructions:

The secret behind this lies in a BUD switch (Block Update Detector), which can be seen here (orange blocks):
This is what it looks like when it is fully assembled, i.e. functional:

This is what it looks like when it's activated:

Both pistons are not sticky.
First of all, it is best to connect the mechanics to it first, which will then destroy the flare (purple blocks):

Next comes the actual door, which is also connected to the BUD switch (green blocks):

Last but not least comes the piston that resets the BUD switch (blue blocks):