How's cv::DescriptorMatcher::match working in detail?

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I'm looking at some quite confused (at least for me) openCV code and questioning myself how it's working.

The question starts here.

The link shows the implementation of

BOWImgDescriptorExtractor::compute

which uses the function

DescriptorMatcher::match

at line 181.

The calculation time is quite fast and I'm asking myself how openCV archives that? How does function match links the nearest cluster to a given descriptor? In detail, how is std::vector< DMatch > matches getting filled so fast?