In Jazz music (and in all it's descendant forms of music) the 2 & 4 are accented. They are the weak beats, which recieve a sharper sound to them to make them stand out a little more. Swing dancing is based on African ideas of rhythm and body movement and European ideas of partner dancing, combining together in a uniquely American way. The further away from the vernacular the dance, the less African influence the more European. The value systems while not necessarily incompatible, do have some conflicting precedence. One must be given priority. The ballroom swing dances favor the European value system, and reduce the African influences. East Coast and Jive tend to have the strongerAfrican influences compared to other Ballroom swing forms (boogie woogie, bugg, double bugg, rock and roll, acrobatic rock and roll &c.). West coast Swing is not technically a ballroom swing form, but has distanced itself from its original African influences, though is still far closer in general than the aforementioned dances. Because of the purposeful distancing of African influences certain defining elements are reduced... namely the ability to swing. Since the patterns tend to follow a more even time frame, with little push and pull of timing, they have resorted to a more artifical (or studied if you prefer) means of creating tension within the rhythms used. Emphasizing the weak beats with the step on the same beat, rather than using the African originated manner. The African, and by necessity, the vernacular jazz form of dance rhythm give emphasis with a visual rhythm and by using the floating second beat of the triple. The visual rhythm component is created by the continous downward bounce on the whole beats of the music (1, 2, 3, 4), and by placing the second step of the triple closer to the third step in timing it puts a hang or hover in the downward movement. This adds a subtle emphasis to the even beats of the triple steps. The floating beat idea is a little harder to understand. If you were to start a metronome at the begining of a Swing song (let's say "It Don't Mean A Thing" in deference to Joe) so it hits even with th efirst two beats of the song... what you'll notice is that the instruments will "slip" in timing away from the meter being ticked out, sometimes falling behind, other times racing ahead. Vernacular Swing dancing does this also. The instruments and the dancers are aware of where they lay in the music, but don't worry about their second or third beat being early or late. What matters is the overall phrasing. This is easy enough to demonstrate in person while dancing but difficult to explain through this medium. The best way I can get someone to understand is to take a Swing song (or a swinging song, since Swing is a specific genre of jazz, but all jazz songs after 1918 swing, at least until you start reaching the avant garde era) and pick a single instrument, piano, trumpet, trombone, sax, guitar, vocal, and clap out every note played while counting out the measures. What you will notice is that they will begin and end their own phrasing in a slightly different sync than that being marked out by the bass and drums rhythmic displacement. Sometimes starting their phrase just before the 1 is picked up other times as late as the four and ending on the two of the next phrase.