"as though it were a program that could respond without delay. I wonder, how is that done."

Nothing strange about it. Sum. Every brokerage has a dedicated trader who handles all trades for them, nowadays thru a computerized trading platform. He can adjust the program momentarily to reflect the action on any given day, to do trades of 100 at a time or 100k, and usually does so in order to not disrupt the market. He simply puts up an offer of, say 1000 shares at a given price, with the computer programmed to replace the offer immediately when filled. This is actually designed to prevent manipulation, since no one but the trader knows exactly what he is doing, as you observed.

Best regards

K