Retraction has 2 effects which both make a large impact to stringing. 1) removing extrusion pressure and 2) creating suction at the tip of the nozzle. The #1 effect is seen immediately after a swap. The #2 effect is seen shortly after a little bit of extrusion, and to account for that we change the slicer to infill before perimeters. There is also the wiper on the cutter arm (not seen in this video. More info on Kickstarter page.) that can be used for priming the nozzle.