From the video, it could be seen that Zhai Jing was originally quite a distance away from Le Wan. The fact that he could rush over so quickly and grab hold of her in time meant that Zhai Jing must have been paying attention to Le Wan's movements at all times. That was why he could react so quickly when something happened.
Moreover, from the camera, it could be seen that Zhai Jing's eyes had been fixed on Le Wan and had not moved away for a second. After Le Wan fainted, his face was filled with panic and anxiety.
Looking at the image on the phone, Le Wan finally understood what had happened after she had fainted. Her fingers brushed across Zhai Jing's eyebrows and eyes on the screen. The Zhai Jing in the camera was obviously franker than the one standing in front of her.