On an Angel of Water, the nozzle only comes out when you release if you are not positioned correctly on the machine. When you have inserted the nozzle and started the flow, you need to scooch down until your thighs are in full contact with the machine surface. In this position the nozzle is into your rectum roughly 6 inches and doesn't come out even if you pass a big lump of poo. Even if you try, expelling the nozzle is not possible.
If you do this, the only reason the nozzle will come out is if the therapist has made the flexible connecting tube between the machine spigot and the Yankauer nozzle. The 'official' one is about 5 inches long and is made of ribbed tubing.
As to why one would take a ride on an Angel, there are at least two reasons:
A session is a very relaxing experience. I almost go into a dreamy trance. The warm water flow is very soothing and relaxing.
The room has subdued illumination and soft music and the position is comfortable. The sounds of the water flowing also relaxes.
The other reason for an Angel ride is severe constipation. A session is far more gentle and comfortable that a mighty enema or powerful laxative.
Once you are on the machine and the water is flowing, even the hardest poo starts to break up without any effort or straining.
After 15 minutes or so of releasing small lumps a large, effortless poo just happens.
A therapist friend has told me that there are several clients with kids that chronically withhold, so they bring the kid for an Angel session. The kid just is on the machine, playing a handheld game, and really doesn't notice they are pooing.