Interpolation is a type of mixing, but it's different from audio or video mixing: it's similar to "tweening" in animation, which is shorthand for "generating in-between values." In Whorld, the crossfader is used to interpolate between two patches. The interpolation affects the parameters and their oscillators, as shown below:

Oscillator WaveformSwitches halfway
Oscillator AmplitudeLinear
Oscillator FrequencyLinear
Oscillator Pulse WidthLinear

Each variable has a value in patch 'A', and a corresponding value in patch 'B'. If the two values are the same, crossfading has no effect on that variable. Otherwise, moving the crossfader from left to right changes the variable continuously from its 'A' value to its 'B' value, such that the variable is halfway between its 'A' and 'B' values when the crossfader is in middle. This is linear interpolation, and it's used for all variables except the waveforms. The waveforms don't change continuously: they switch from their 'A' value to their 'B' value when the crossfader reaches the middle.

For example, if Ring Spacing is 5 in patch A, and 10 in patch B, the output Ring Spacing will be 7.5 when the crossfader is in the middle.