SAM
           What happened to your hand?
                         
                          SUZY
                          (PAUSE)
           I got hit in the mirror.
                         
                          SAM
                          (TAKEN ABACK)
           Really. How'd that happen?
                         
                          SUZY
                          (SHRUGS)
           I lost my temper at myself.

The editor of the film, Andrew Weisblum, is frequently nominated for editing awards, including Moonrise Kingdom. Apparently the strength of the (in)famous beach dancing scene is entirely a creation of the editing process:

Anderson’s Moonrise Kingdom is a love story, but convincing the audience that the 12-year-old protagonists were, in fact, falling in love was a challenge. Weisblum says of the young leads, “My impression is that they didn’t have any particular interest in each other.” He carefully sculpted their performances in the edit, piecing together audio from multiple takes (including recorded rehearsals), and using music and visual stylistics to heighten the feeling. In one scene, standing at the edge of the lake where they are camped out, the young runaways play a French pop song on a toy record player, dance in their underwear, and have their first kiss. Anderson had planned to let the scene play out awkwardly in a master shot, but it wasn’t working, so the camera crew “ran around handheld,” getting close-ups and shots from every which way. In the end, Weisblum says, he used “every angle to shape it…and two thirds of the dialogue is not from the day.” His thinking in the editing room was, “It’s this incredibly strange, crazy scene, so let’s just embrace how bizarre it is.” The result is a surreal set piece that plays like a lost flashback from Pierrot le Fou, and captures all the discomfort and tenderness of first love.

Moonrise Kingdom uses both J  cuts and L cut to tighten the editing and enhance the emotionality of scenes. At about 5:20 in this long tutorial on using the Precision Editor is how to do both cuts in iMovie: