Unison - Detune
Simultaneous voices singing approximately the same pitch, but slightly detuned from one another. Inspired by the sonic artifact produced when a choir sings a note in unison without being in perfect tune with each other, this characteristic can be replicated by duplicating a single audio recording and moving the pitch randomly up and down within a certain pitch range. The more voices and the wider the pitch range, the more dramatic the effect. Also, playing simultaneous, but separate recordings of the same singer will naturally produce this effect since he/she will not sing additional takes in the exact same way.
Since you have simultaneous voices from the unison effect, panning each voice a little to the right or to the left will increase the "width" of the background vocals, yielding a wide stereo image.
In the provided example, the voices are harmonizing, with the most pronounced interval being the perfect 5th. This interval is special because it is one of the lowest harmonics in the overtone series and, as such, is so consonant with the fundamental pitch that it can be perceived as one and the same; however, it adds a second set of formants and beating patterns that makes the sound pick up a stronger and more aggressive timbre. In addition, voices can be "doubled" an octave above or below. This will also work to make the sound "taller" as it occupies a wider band of frequencies within the human range of hearing.
The result is very powerful because it occupies a large band of frequencies (thick/tall), doubles on very highly consonant frequencies (5th, octave), and has wider spacial imaging (stereo).