On the issue of soap v. glycerin, there are difference. While both will make someone poo strongly, they act differently.
The enemas I'm comparing here are:
A 1500ml warm water enema, given with a bag, with solution being warm water with Ivory or Castile bar soap swished in until the water becomes noticeably white.
A 1500ml warm water enema with 100ml of pure, liquid glycerin thoroughly mixed in. IMO more is pointless and does little.
Both would be given with the recipient in the same left side position with the bag hung at the same height.
The soapy enema can usually be fully administered to the recipient, especially if constipated, and some time will elapse after the solution is all in before the cramping starts. Soapy enemas are usually given with slow flow rates for recipient comfort.
With the glycerin solution, the ramping starts soon after the solution begins to flow, and it is quite often a real challenge to just get the full bag into someone. Glycerin enemas are usually given with fast flow rates but that can also cause premature cramping.
I don't think either enema is much more 'slippery'... to make poo passage easier... than the other.