This is only for self-given enemas, I'd love to have a wow-list when I'm just the taker, but that hasn't happened yet.
1: First contact. The lubed finger touching my anus to grease it up and massage it a bit to relax
2: Nozzle entry. The anus opening around the nozzle, feel of the nozzle sliding in, and for a bulged "retention" nozzle the feeling of the anus coming to rest at the narrow part.
3: First rush of enema fluid. 'Nuf said.
4: When the colon opens up and the fluid progresses beyond the rectum, with both the feelings of it going higher and the reduction in urgency. If this doesn't happen I know it's not going to be a very nice enema.
5: When you finally relax your anus and the enema starts to rush out. Combination of the feeling of relief, rush of the water and solids through your sensitive anus, and that initial feeling of surrender to the enema - that the struggling is over and you're just giving in (especially if you've done a retention period).
6: When you realize that the expulsion is over (if it's a good expulsion and everything comes out pretty quickly), and you know you're feeling better and all's right with your ass (for now).
Sometimes (depending on mental state, physical state, and enema solution/quantity) that first cramp is nice, too. You know the enema is working, you know it's "game on", and while you know you're going to lose eventually (see #5) the fight to retain as long as possible does have a certain satisfaction.