Traits my friends, traits. This is a beautiful pocket that McCarthy turns around and finds after faking the handoff. It's a one read situation, the play is designed to go to this guy (who ends up scoring the TD). But what does McCarthy do in this play?
While faking the second handoff (sweep), he notices DE #33 swinging wide on the RT. He notices this while spinning and reorienting himself. McCarthy feels that pressure, but he also notices he has space in the pocket, so he steps up, because the WR is not yet open. he steps up, doesn't panic, and delivers a strike.
A LOT of QBs panic there when the WR isn't immediately open. if they sense/feel the pressure behind them. In that moment, it appears that there's some gaps that open up. See:
That is tantalizing grass, especially for an athlete like McCarthy. Only a millisecond later, the Dlinemen disengage and the gap is occupied.
McCarthy kept his eyes downfield, moved up in the pocket, and delivered.