VFE grounds one side of the LDR output and uses the other side to essentially ground-and-release the Speed or Depth control via an on/off/on switch. Check out the Old School Trem doc for ideas/details.
I've had good success using this with Phasers, Chorus, Flangers and Trems.
The buffered out goes to the effect input.
The two ENV OUT go across the control you want to influence with playing dynamics. (in most cases one ENV OUT is simply grounded, the other ENV OUT goes to the outer (input) lug of the "influenced" control)
1978 Gibson Les Paul Standard, Cherry Sunburst