Good question,i never use it,i do non precision the old fashion way,CDA with altitude call outs and a VDP to MDA.
The aircrafts lateral postion comes from ADIRU postion inputs.
On the NG,it comes from the GPS,if the GPS fails then the IRU will provide lateral position to the FMS.
The vertical path profile is coming from the FMS,so proper QNH setting and temperature correction should be used in order to follow a proper path.
Its a good aid in challenging terrain but standard brief,call outs and navigating should be maintained,it is not a tool designed as a replacement for basic piloting skills.