When using a VFD in combination with for example a lathe, if you wish to slow down the main spindel at a fast rate. Then this brake resistor is required. It turns the mechanical energy into heat. Also for safety we recommend it. Some motors can take 10-20 seconds to slow down. If anything happens, you are always too late then. With the brake resistor its decreased to about 1 second. Do keep in mind that not every mechanical system can slow down this fast. It could cause damage to the mechanics. You can program what time the system needs to decelerate. On our 24k TeknoMotors we set it to 3 seconds for example.