Hmm, this is actually a pretty good update. Rapid jabs by and large have been really bad in all previous smash games (a few specific ones, like Falco's in Brawl, are kinda the exceptions that prove the rule). The main reason is that they tend to be really unsafe to use even on hit; you don't have a good way to transition out of rapid jabbing to some safe and advantaged position. These finishing moves could very well offer that, and that's a huge boon to this style of move. Mr. Game & Watch in particular could be wildly improved here; his rapid jab was so awful in Brawl, but it sure would have been really useful if it were just safe to hit with since he desperately needed an option like it that could be thrown out quickly at that short range and jab1 -> nothing was not exactly what I had in mind (his jab1 was pretty good, but the rest of his moveset worked poorly with it).
In general I like this too since it shows a thoughtfulness of design. A large class of moves that have been bad previously are getting a thoughtful upgrade to fix their main issue. This is promising for the general design direction of this game whether you care for these moves or the characters that have them or not.
EDIT: As per how they work, it's pretty simple to imagine. You do rapid jabs in Brawl by holding A. I imagine the finisher just triggers when you let go (or stop mashing long enough) for the move to end. Instead of going into the ending lag, it goes into the finishing attack. Otherwise these rapid jabs wouldn't actually be rapid jabs but just multi-hit ordinary jab combo enders...