A mini cut is a short, deliberately somewhat harder diet phase in the middle of a muscle-building phase, typically lasting 3 to 6 weeks. Afterwards you go straight back to bulking.
The term comes from gym practice, not from research, and the length is a community convention rather than a measured value. What you can derive from studies is how fast a short cut should go and how to protect muscle during it.
When it makes sense
Typical reasons are a bulk that ran faster than planned, wanting to extend a bulk without body fat climbing further, or a fixed date a few weeks away. It makes little sense if you are already lean and your bulk is on track. If you want to lose several kilograms of fat, a regular cut over several months is the better choice.
How hard it can be
Helms et al. 2014 recommend about 0.5 to 1 percent loss per week for dieting bodybuilders. For a mini cut the sensible rate sits at the upper end, around 0.7 to 1 percent. Much faster tends to cost muscle. A meta-analysis by Murphy and Koehler 2022 suggests that little new muscle is gained in a larger deficit, which is exactly why the phase stays short.
Example: At 85 kg trend weight and 1 percent per week, your trend sits at about 81.7 kg after four weeks. That is roughly 3.3 kg, a correction rather than a transformation. These numbers are a worked example.
Steer by the trend
The first week is misleading, because fewer carbs mean losing glycogen and water. Judge your rate from week 2 onward by your trend weight. When you switch back to bulking, the scale jumps for a few days, and that is water too.