mechanic in the 'a'
 

2 person mechanics.
U1 in the 'A' no runners on base.
Pop up to the first baseman, ball coming down right where the U1 started. Where should the U1 be going?

Bust inside and let PU take it.

Toward your same normal spot for a play at first ... watching F3 (and maybe F4) to ensure you don't get near them. The catch is not your call, get in position for the play at first if it's not caught.

It's your call if you choose to let the PU take the ball on the line. You can also let the fielders clear you and take it yourself. It's a risk that you'll be in the way, so the prudent thing is to get out of the way.

You can't just set for a play at first, though. If the ball falls, you have the play at second. As Matt said, bust inside.

For OP popup, as well as most infield popups, "Popup equals pivot".