Think of it like a Manual control in an MXR, or Filter Matrix in a Mistress. It is the base delay time in a flanger.
The LFO sweeps around; centered on this short delay time (only milliseconds). Or, you can eliminate the LFO influence, and use it ‘manually’ to sweep through the small range of delay.
I believe it was also called Modify O/P (or similar) in some variation on the theme. The Instant Flanger has a unique feature, where the delay time is controlled by the oscillator, envelope follower, manual, or any combination of that.
If your have an expression pedal, try mapping that to Delay Offset. Flanging under foot control.