Stupid q re telling instead of showing...

The trick is in the transition from the present event to the past event. You need to make it clear to your reader that what follows is a memory, then even though it is in present tense, the reader will know it takes place in the past.

To keep yourself from writing this in the past tense instead of the present, don't picture yourself as the character recalling a memory, but instead treat the memory as an entirely separate scene from the one it's in, and write as though it was happening in the present time. You can tweak it in revision later to make it a more seamless transition.

Is it a fully fleshed out scene played back as a memory? Can't you write it in the past tense as long you're making it clear it's a memory that happened in the past? Maybe I'm confused but I've definitely seen this in published novels and have used the same device in my own writing. I'm curious to see what others think.

I think you can choose either tense, whichever fits your story better. Relating it actively is more like a flashback than a retelling of the event.

Retellings are hard to stick 'show' in; I agree. I do think they can be clever and a great way to do it, particularly if a summary of the event fits best. Whatever makes it the most interesting and fits best, in other words

I think telling pov is often easier to write than showing, alas.

I'm actually surprised to see you hoping to write a past memory in present tense. For me, the location of the primary POV narrator dictates what tense I use. If the primary narrator is talking about current events, it's present - if past events, it's past. So long as it's clear where the narrator is, and what the difference is between those two types of events, I don't see a problem with past tense.

As far as the telling versus showing thing, I'm not clear on what this piece is, which makes it hard to judge. Is it a freestanding scene? A dream? A flashback? Or is it a recollection that impinges on the current consciousness of the narrator? How you treat it (and the tense too) will depend on which one you choose. A piece of text that looks like obvious "telling" can also be "showing," if the way it is told reveals something about the identity or state of mind of the narrator, for example.

I have a discussion of showing and telling on my blog, which compiles four different meanings for the distinction and tries to show the differences between showing and telling in each case. It's here, if you feel you'd find it helpful.

Phew, Juliette said exactly what I was thinking, but far more eloquently.

Don't take the "rules" as gospel. Perhaps it works perfectly for some reason. You won't know until you've given it to an actual (beta)reader.

Fenn, it's hard to judge without seeing the context. You can have the narrator do something like compare his/her current situation to a past experience, which will let you just hint at that past and make people curious. That's different from laying out all the details of the past event as though the narrator were narrating a scene. Dialogue isn't what makes a scene feel active, though some of it can help. I'd say the motivation of the main character, his/her goals and what is at stake, are what provides a sense of drive in a scene.