Yes, I am going to reply to my response:
Talking about chase mechanics,
here is a good example here from USA softball May clarifications.
One, using my "watch the fielder theory" U1 looks to be watching F9 instead of the flight of the ball. That's good, because it puts him in a position for two elements of the play: fair/foul and catch/no catch. When you SEE the fielder runner towards the line, it is important to straddle the line for this view.
However, there is a second aspect that is stressed on the USA softball DVD's: stop-see the play-make the call (and if you can't here that with a deep, Southern draw, you need to watch the DVD's). U1 was not
set to see the play (in this case, a first touch in fair territory).
BTW, U3 needed to be inside to cover first, assuming that no runners were on base, and second and STAY inside to cover third (and not be behind the play).