Dedee, you are very kind, but I do not consider myself an expert. The new frame is first in sequence, because in the second frame the left leg of the POI/BG is further away from the platform in the back right. He is in the process of lifting his knee to stepping forwards:
From here he seems to be making a left turn. His left foot is past the point of where his right foot is still planted during frames one and two:
We only see three frames. IMO we are looking at app .5 +-.1 of movements. Film and Hd frame rate is about 25 frames per second. Half a second would be equal to app. 12-13 frames. Those missing frames would be accounting for any further movements. The three frames we have may have several frames in between them.
All IMO
-Nin