updown~ is driven by a ramp from 0 - 1 or 1 - 0. It generates a trapezoidal function where the attack and release times are independent of the speed of the input ramp, unlike trapezoid~ where start and end times are specified as a proportion of the total input ramp cycle and thus will vary with the speed of the input ramp. Note that the up phase of the updown~ object's output occurs after the reset of a phasor or a zero-to-non-zero transition, whether the input ramp moves up or down. The down phase of the output occurs in the time before a reset (or the end of the ramp) is expected to occur.