I'm so glad to see his tenacity paying off, it's been great to watch his progress. To be honest I never thought this would be possible because there's just so much variance with these small solid fuel motors. Impressive.
The mechanism to throttle the solid rocket motor is simple and ingenious - ceramic "jaws" underneath constrict to block a portion of the exhaust plume (an idea borrowed from Josh at Simple Robotics). Here's a link to the juicy bit in an earlier video on it, although the whole thing is worth watching: https://youtu.be/8sksiYfkses?t=658