[Cbc] What's CBC doing when it reduces rows and columns?

[Nathan Petty]

During a solve of my MIP using CBC (2.9.9), I see the messages "Full
problem 3323 rows 656910 columns, reduced to 1169 rows 832 columns"

What exactly is happening there?  Is it doing an internal column
generation where a subset of the problem is being solved and then
reduced costs are being used to determine which are coming into the
basis?  My gut feeling is that is not doing this and it is instead
executing some heuristic, freezing variable values or something,
because if it was truly doing column generation the resulting smaller
problem would solve much faster than it is.

I am actually interested in custom coding my own column generation sub
problem, so if it's already doing it, and its slow, it's a great
disappointment to me.

Thanks,
Nathan