"A helicoil is spring steel AFAIK and screwing it in from
the front will tend to expand it, screwing it in from the
rear will tend to contract it making it go in a lot easier.

An earlier poster stated you needed a special tool to
hold the coil in contraction * to insert it, I'm sure he
is right, its goes in far easier the other way AFAICT."

A thread has the same handedness regardless of which side you are looking at. The insertion tool has a mandrel which catches a tang that goes across the open end of the Helicoil. This twists it tighter in the direction in which it is wound which causes it to effectively shrink so it will go into the new threads. When the driving force is released and the tool is backed out the helix re-expands and locks into the hole. The tang is then broken off (it is notched) and the Helicoil threads can be entered from either side.