The figure.
How much to thin, in [0, 1].
The meter the figure is counted in, in any form that names one. The ranks are the meter's, so a full turn of the dial leaves the downbeats of the bar the figure is actually in — which is why it is asked for rather than assumed.
The surviving events, in the order given.
Drop the notes carrying the least of the metre.
The easing direction: at 0 nothing is dropped, and as the amount rises whole ranks of positions go, off sixteenths first and the downbeat last. It is a ranking rather than a sampling, so the same amount always removes the same notes and a caller can explain which ones went.