I've been reading many questions about why tuning fork has two prongs. Many of them explain that each prong oscillates with opposite phases. As a consequence, the compression and decompression of air gets stronger and the oscillation on the stem is reduced, which results in a "purer" sound and less damping by the stem (this was what I understood).
However, I couldn't figure out how this opposite phases are created. When I hit one of the prongs, it will start oscillating with a phase. Then this oscillation propagates to the other prong because it's in touch (connected) to the first. But why it will have a different phase? Also, I've been wondering that, if this phase difference reduces the oscillation on the stem, how the resonance is going to happen strongly when I touch the stem in a resonance box?