There are different kinds of hobbs meters.
Some just count engine revolutions, and are calibrated to read hours assuming say 2300rpm. I think most GA types are in that category.
Some read time but only if the oil pressure is over X psi.
Some read time but only if the rpm is over say 1200 (mine does that).
Some are purely electric, and people have been known to turn off the master switch during flight to reduce their bills
