That code is a “band-pass filter” (https://en.wikipedia.org/wiki/Band-pass_filter)
The lower frequency limit is defined by “hpfreq” (which I presume is an abbreviation for “high-pass frequency”) and the upper frequency limit is defined by “lpfreq” (presumably “low-pass frequency”).
The frequencies of the filters are control signals, produced using PWLV (see: http://www.cs.cmu.edu/~rbd/doc/nyquist/part8.html#index394). A you see in the manual, the values inside the PWLV command are time/value pairs. So for example, for the high pass frequency:
(pwev 100 0.4 100 0.9 10 1 10)
at time = 0, value = 100
at time = 0.4, value = 100
at time = 0.9, value = 10
at time = 1, value = 10
As you see, during the time period 0 to 1, the frequency falls from 100 to 10.
In Nyquist “effects”, a time period of “1” is the duration of the audio selection.
To make the frequency rise or fall in whatever way you wish you need to set the time/value pairs to suite your requirements.
Example: to make a band-pass filter with a 2 octave band-width, where the centre frequency rises from 40 Hz to 2000 Hz then back down to 40 Hz: