When Little Mac completes his Jab combo on Fox with that uppercut, the streaks of red that Sakurai mentioned will show up whenever a strong hit is dealt (with the red color due to Little Mac being Player 1 for that moment) appears for a split second. It's the only time in the trailer, aside from Mac's OHKO uppercut, where one of Mac's attacks launched someone hard enough to create those color streaks. Maybe Mac's Jab combo finishing uppercut can be a solid KO move at a certain percent.
Also, Little Mac's flurry of jabs before that uppercut are shown to have the benefit of keeping the opponent in place for a while, at least. Could have some use in Team Battles, where Little Mac's teammate can come in and deliver a KO move while the opponent is being beaten mercilessly by those punches.