What does KRT mean in this diagram?

I am working on building a reverb. This article explains how to build a reverb pretty well, but I do not understand what the KRT in the main diagram stands for. The article says The term Krt controls reverb time, but what does the actual block do?

• I guess it's just a multiplier. – Matt L. Jun 19 at 16:13
• After implementing the rest of this, I guess it's pretty clear that you are correct. – popctrl Jun 19 at 17:18