wild animals running away
The stimulus starts with a hunter entering a forest and making noise. The safety of any animals closeby, like a deer, is significantly lowered. The stimulus triggers the set reaction, in this case it is a deer trying to increase its safety.
The receptors would be the ears of the deer hearing the movement of the hunter. The receptor receives the information from the stimulus and sends it to the control center.
The control center would be the deer's brain. The deer would realize it is no longer safe like it was and must take action to increase its safety. The control center compares previous conditions to the current one.
The effector is the deer running away to be out of range from the hunter. It is now safe from danger. The effectors change the condition that something is currently in to achieve balance.
Why is it negative?
This mechanism is negative because the stimulus decreases something, in this case safety. The effectors then react from the process to counter the effect of the stimulus to create balance. The deer runs away to be safe again and has balance restored.