I generally run at 65-75% power, with whatever fuel burn keeps the mags from
loading up. And if I'm flying something with a constant speed prop, I try to
run the power at whatever combination will allow the slowest RPM (for reduced
noise). 2100 RPM is quite a bit easier on my ears than 2400 RPM.... and I can
still pull 65% power if I don't go too high.

What does "mags loading up" mean?

Thanks,
John