If u have ever gone to a lab to take a blood test, they will often have a version that looks similar to a blood pressure cuff, which allows the Nurse to control the level of blood flow restriction while taking the blood.
with traditional strength training, u need a heavy load (at least 60 to 70 percent of your one rep max) in order to make your muscles stronger and bigger. With a BFR band, you're able to achieve the same effect with a much lighter load.
For example, if you would normally have to perform squats with 12kg weights in order to increase ur lower body strength and muscle size, with the use of BFR u would only need to use about 3-5kg weight to achieve the same level of strength and muscle growth.
A BFR BAND tricks ur brain to think u are lifting heavy by reducing the blood flow, but without having to actually use heavy weights.