The reaction mechanism for complex III (cytochrome bc1, coenzyme Q: cytochrome C oxidoreductase) is known as the ubiquinone ("Q") cycle. In this cycle four protons get released into the positive "P" side (inter membrane space), but only two protons get taken up from the negative "N" side (matrix). As a result, a proton gradient is formed across the membrane.  In the overall reaction, two ubiquinols are oxidized to ubiquinones and one ubiquinone is reduced to ubiquinol. In the complete mechanism, two electrons are transferred from ubiquinol to ubiquinone, via two cytochrome c intermediates.
Overall:
* 2 x QH oxidised to Q
* 1 x Q reduced to QH
* 2 x Cyt c reduced
* 4 x H released into intermembrane space
* 2 x H picked up from matrix
The reaction proceeds according to the following steps:
Round 1:
# Cytochrome b binds a ubiquinol and a ubiquinone.
# The 2Fe/2S center and B heme each pull an electron off the bound ubiquinol, releasing two protons into the intermembrane space.
# One electron is transferred to cytochrome c from the 2Fe/2S centre, whilst another is transferred from the B heme to the B Heme.
# Cytochrome c transfers its electron to cytochrome c (not to be confused with cytochrome c1), and the B Heme transfers its electron to a nearby ubiquinone, resulting in the formation of a ubisemiquinone.
# Cytochrome c diffuses. The first ubiquinol (now oxidised to ubiquinone) is released, whilst the semiquinone remains bound.
Round 2:
# A second ubiquinol is bound by cytochrome b.
# The 2Fe/2S center and B heme each pull an electron off the bound ubiquinol, releasing two protons into the intermembrane space.
# One electron is transferred to cytochrome c from the 2Fe/2S centre, whilst another is transferred from the B heme to the B Heme.
# Cytochrome c then transfers its electron to cytochrome c, whilst the nearby semiquinone produced from round 1 picks up a second electron from the B heme, along with two protons from the matrix.
# The second ubiquinol (now oxidised to ubiquinone), along with the newly formed ubiquinol are released.