Micro drive is a type of bicycle drivetrain, mostly BMX and MTB, that uses smaller than standard-sized sprockets.
[1] The smallest rear sprocket that fits on a freehub body is a 10 or 11-tooth, but with the use of a cassette hub, sometimes called a micro drive rear hub, sprockets as small as 8 teeth may be used.
[2] Some examples of micro drive hubs are Shimano Microspline (12-speed, 10-teeth) and SRAM XD/XDR (11/12-speed, 9-teeth).
The advantage of micro drive is that it means a smaller front sprocket, or chainring, can be used without affecting the gear ratio, providing better ground clearance.
As the front and rear sprockets become smaller, the tension in the chain increases, and stretching and breaking can occur more easily.