BLDC tool with mouse arrow left right

I am using the BLDC tool to test my FocBox VESC. The motor is RC car Traxxas 1/10 Slash 4x4 Ultimate 3500kV VELI BRUSHLESS MOTOR, 13T PINION GEAR. With the BLDC tool open on a linux computer, i found that if I press my keyboard left or right arrow, the motor will be activated. However, left and right are slight different. If I press left key, the motor will rotate backwards. If I release the key. The motor will stop. If I press right key, the motor will rotate forwards. If I release the key, however, the motor will continue to run, but at the slower and slower speed, and then stop. Anyone know why?