HIMYM has built up so many gags over the years that it can pull them out from time to time with great effect. Ducky tie, slap bet, Barney and Robin--lots of old favorites in one episode. "Disaster Averted" moves very fast, as Ted tries to tell a story about boogie boarding which wanders off into Hurricane Irene, with the majority of the story spent showing how fun the group is together. Between Ted telling the story, Barney wants to get rid of his tie, because he'll be meeting Nora's parents. Barney trades three more slaps on top of the one Marshall already had--and promptly receives two slaps. This leads to Barney and Robin kissing in the cab, something that should drastically alter what's been going on this season, in particular with Nora.
The episode is odd because it's almost entirely comprised of a flashback. It's some time in the past, probably near the beginning of the season, but we not know exactly how it fits exactly into the show's timeline. Once the episode starts rolling, however, it's more of an afterthought when the events occurred.