How to: crescendo/descrescendo with just 2 buttons?

On the organ of Glebe I saw the improvisation genius Wolfgang Seifen (via youtube) implement an increasing crescendo and/or decrescendo by just hitting the same crescendo button or decrescendo button twice, which would provide me with an excellent tool in especially improvisations.

How can this be implemented????

Thanks in advance!

Do you have a copy available of that YouTube video?

Rgds,
Ed

He is using the incomparably flexible stepper system provided in Hauptwerk. It provides for up to a 1000 step sequence. This is covered in the very thorough user manual on pages 90-94. Perhaps unique (?) to the HW system is the ability to access intermediate points of the sequence directly.