To some,all very confusing this 'forward' and 'side' business from over the pond!
As I see it you'd be hard pressed not to be going sideways when slipping through the air - try it with the windows open, chaps, it gets windy on one side or the other! So the term 'forward' slip is bound to be a little inaccurate as you can't actually do without the sideways bit and still be slipping.
Presumably, the term 'forward' is trying to describe a slip where the flight path and ground track coincide with the extended centreline of your runway or airstrip (since all slips that aren't turning,by definition, have a constant direction over the ground ie track - wherever that may take you.. ).
Slipping turns are simply slips that have insufficient rudder applied to prevent the machine from changing heading.
Read your Biggles
PS shocking that anybody could confuse crabbing with slipping...