It's all about fingers (apparently!)
They measure how far out of the engaged position the baby's head is in "fingers" or fifths. So if you are 5/5 engaged, the babies head is not at all in the pelvic area, the midwife can measure 5 fingers widths of his head still out of the pelvis.
As you get to 4/5 engaged, she can only measure 4 fingers out of the pelvis, so 1 finger (or 1 fifth) is IN the pelvis.
Then by the time you get to 1/5 engaged she can only measure 1 finger out of the pelvis, so 4/5 is in the launch position ready to be ejected!
Apparently!