Override interface allow to speed-up or slow-down the effective engraving speed (as well laser power) in real time during engraving process.
Speed overrides is a new feature of grbl v1.1, so it not available in older grbl version. This feature was implemented in grbl in a tricky commands interface:
0x90: Set 100% of programmed rate.
0x91: Increase 10%
0x92: Decrease 10%
0x93: Increase 1%
0x94: Decrease 1%
LaserGRBL try to made it simple, like setting value in a slider control. A background worker thread send the right number of increase/decrease commands to reach settled target.
Note: this feature works only if you have not modified overrides section of your grbl config.h file.