Not necessarily. It is not a function of the TB6600, but it is a function of the stepper motor controller design.
When the TB6600 is requested to reduce torque it will reduce the motor current to 30 percent of the current limit value.
As an example, the X-controller has a user selectable reduction of torque based on the fourth switch in each micro-step selection DIP switch. When the switch is ON (from the pictures of the board I have seen it comes from Inventables set to ON) the X-controller will request a reduction of torque when there is an absence of step pulses for that motor. When the switch is OFF, no reduction takes place.
Each stepper motor controller that uses the TB6600 can elect to use this feature or not. It just depends on the external design.