Optical flow sounds very similar to motion detection. I am not an expert by I’ve found a few motion detection libraries on the Internet that I’ve been able to use to build my own programs.
Basically motion detection compares two or more frames of an image to determine if there are any differences. It sets a threshold for variances, so if the differences are above a certain threshold then that means that motion has taken place.
To extend the use to optical flow applications, once the computer identifies that motion has taken place, it can then define boundaries for the object and begin tracking it on a frame by frame basis. I think they use this technology in security cameras in addition to the other applications mentioned in this piece.