ok so its my understanding that coast stops pushing the filament out "x" amount of distance before a retraction or maybe a change in in direction, im not sure which. coasting allows the excess pressure in the tip to be relieved so even though the extruder isnt pushing the filament out the plastic doesnt continue to ooze. kinda like when you release the pressure from a caulking gun. the retraction pulls the filament back a bit during a "rapid" move to prevent ooze during the jump from one side of the part to another. i do believe the coast only happens on the rapids as well, not at the beginning and end of a line. for instance when zigzagging back and forth for infill, the starts and stops wouldnt apply to coast or retraction.