I initially responded to this thread waaay back in 2013, but an update is in order.
Now, with my collection of nozzles from Silicone Nozzles, the act of inserting a lubed finger serves not only as a prelude to an enema adventure; but it also heightens the erotic feeling of the slick silicone gliding through the anus and into the rectum. When I give hubby an enema, a lubricated finger inserted into his rectum has a calming effect and makes his anus more receptive to the nozzle that awaits. For me, it's the pressure relief valve that allows for the pent up tension to dissipate just a little before the enema begins.
It's definitely a part of our enema ritual that serves as a little subconscious reminder that the fun is about to begin!
Have fun and play safe!