SFD is the 8th byte in the Ethernet frame. This byte reads 10101011. Essentially, it changes the final bit to a 1. Therefore, after 62 alternating bits, there are 2 1s in a row. This helps to signal that the preamble is over, and that the frame is about to begin in case any bits were missed (for example, at the start of the transmission).

While the CCTV operator was figuring out the timing, they may or may not have missed some bikes passing by. To help identify which is the vehicle carrying the important guest, they might use something out of the ordinary, like a police horse. Now, instead of counting 64 motorbikes go by, the operator uses the motorbikes to ascertain timing, and then looks for the horse to identify the important visitor.