In a way this is very awesome.
At the risk of pulling a B^U, I think I've figured out what it's saying. Going by the definition of 集中線（しゅうちゅうせん） at http://websunday.net/mezase/smc_p/manjisen/page01.html , it is the same as motion lines, "abstract lines that appear behind a moving object or person to make them look like they're moving quickly." (from the Wikipedia definition at http://en.wikipedia.org/wiki/Motion_lines , as my dictionaries failed me on this)
「しゅうちゅうせん５０ぽんだしたらてをださないんだぜ」 "If you scram so quick that fifty movement lines trail behind you, I won't lay a finger on you." 「だじぇ！」 (slurred) "Da ze!"「あと、あまあまさんをもってくるんだぜ！」 "After that, bring me mister sweets!"「くじゅなじじはしゃっしゃとしてにぇ！」 (slurred) "Shitty old man, do it at once!"
next is at post #19417
「おがあしゃああああん！！ごわいよおおおお！！」 (slurred) "Mommy!! It's scary!!"「は…はりさん５０ぽん…がまんしたら、おちびちゃんにはてをださないやくそく…だよっ！おちびちゃん…おかあさんがんばるよ！」 "If.. if I endure fifty.. mister chopsticks, it's a promise that little one won't get hurt! Little one.. mommy is doing her best!"
Now, for the B^U part. The joke is in the underspecified first comment of Marisa, which literally translates as: "if fifty concentrated lines (which could be motion trails, or, as it may happen, chopsticks) come out (not specified from whom, implied is the addressee), (not specified, implied self) will not hurt (not specified, implied addressee) (expression of emphatic statement)." Jocularity is derived from the statement being both absolutely correct, and, at the same time, having been intended as quite the opposite: the very definition of (verbal, non-sarcastic) irony.
(Apologies for the above.)
And, of course, I forget the most important part: that's the joke.
I'm still not seeing a joke here.
Also, why has nobody done yukkuri loss.jpg yet? I might have to rectify that.
Lines. Chopsticks. Lost_in_translation.
Actually, I made a mistake. はり means needles, はし means chopsticks.