A time-varying delay: audio passes through a delay line whose length is swept, sample by sample, by a control input. The Delay parameter (1 ms to 2 s) sets the longest delay; the control input, which ranges from 0 to 1 like every control generator in WaveWarp, scans the tap between no delay and that full length. There is a second control input for the right channel, so on a stereo board the two sides can be swept independently — leave it unwired and the right channel simply follows the left. On a mono board the second input has nothing to drive and is ignored.
This is the block the classic built from scratch boards are made of. Mix a little of its output back with the dry signal and a slow modulation gives you a chorus (20–50 ms delays); a short delay with feedback gives you a flanger (around 1 ms); a wild modulation source gives you the bizarre pitch-bending sweeps of the extreme-variable-delay boards. The wet/dry mix is not built in — wire a Linear Mixer after it, exactly as the original boards do, so you can see and change the balance.
Each incoming sample is written into a circular buffer sized for the longest delay the slider can reach, so the setting can be changed while audio runs. The control value scales that length into a fractional tap position, and the output is read back from the two buffer slots either side of it with linear interpolation (Moore's expression), so a moving tap sweeps smoothly instead of stepping between whole samples — which is what produces the characteristic pitch-shifting character of a swept delay. Each channel is tapped from its own control, so a second modulator on the right input spreads the pair across the stereo image; with that input unwired both taps follow the left control and the image stays coherent. One sample-frame is processed per tick, in step with WaveWarp's sample-per-sample engine.