There are a number of reasons why you could need help, some of the main reasons are:
Your medical team are concerned about the well-being of your baby during the birth process
Unfortunately, it appears that your baby is not moving as expected out of the birth canal
For some reason, medically you have been advised against pushing during birth
Essentially the process of an assisted vaginal childbirth is to carry out a procedure that mimics a spontaneous birth, yet at the same time poses minimum risk to both mother and baby.
An assisted vaginal birth is carried out either by an obstetrician or midwife, using specific instruments (forceps or ventouse), to assist the birth of your baby.