not a great explanation IMO. front or rear curtain makes no difference in "freezing" the motion. It's purely the flash that takes care of that. it's not like the sensor overwrites or paints over the image with info received later in the exposure. info/light gathered over the entire exposure is simply summed together with no distinction between what was gathered earlier vs. later. all front/rear curtain setting does, is set whether the streaks lead or lag the "frozen" portion of the image. the shot posted of the boy could easily be front or rear curtain. it's particularly ambiguous because there's nothing leading up to his hand to tell if the streaks were done pre or post flash.
here's an explanation:
http://neilvn.com/tangents/first-curtain-sync-vs-rear-curtain-sync/
either method can still get you streaks, and potentially over important areas of your subject. depending on the motion of your subject and what portions you want less streaky, front or rear curtain may work better. if you're interested in the keeping the leading edge of your subject (ie if you want to keep the front of a person's face who head is moving forward) rear curtain will work better. If you want to preserve the trailing edge (ie if you want to keep the front of a persons face who's head is moving backward), then rear will work better.
i've done some concert photography, and some club event type stuff. it's dark (with sudden lighting changes), and your subjects are often moving pretty quickly and erratically. longer shutters aren't that useful and the flash usually overpowers everything anyways so front/rear curtain doesn't make a huge difference. It's been a while but i'm pretty sure i usually set for front. having the flash fire a half second after the pre-flash or af-assist can cause you problems with your subjects dropping their original expression. then you're just catching awkward, unflattering, between-expression faces. my opinion, ymmv.
edit: also looking at the added shots from the event... yes, killing the background in the first shot was a good idea. the first shot looks a lot more like a real venue. if i were the artist i'd very much prefer that over the background shown in the subsequent shots.