when my midwife did mine she said that a fifth equals a finger... so say you lay a finger along the top of your pubic bone... horizontally along your nickers line... thats 1/5 engaged , meaning bubs head would be just poking out of your pelvis...
therefore- 0/5 engaged you would be able to feel no head when you put that finger along the top of your pubic bone as bubs head would be in your pelvis(and you would more than likely have a sexy waddle going on) and 5/5 engaged means you can line up all your fingers along your pubic bone as bubs head is still up there...
did that make sense???
so with 4/5 and an arrow going up the page, the arrow means 4/5 of the head are not in the pelvis, 4/5 not engaged, 1/5 engaged.