Call of Cthulhu - When should a GM ask for a SAN check?

Sounds like a silly question, but...

“The more you know, the more you know you don't know.”― Aristotle

In CoC, when PCs encounter something horrible or unexplained, the GM asks for a SAN check.

But do you describe the horribleness first and then ask for the SAN check or do you ask for SAN check and then describe what they see?

Sandy Petersen was a proponent of the former: Describe what the PCs see and then ask for the SAN check.

He said this at his Creating a Horror RPG Scenario the Sandy Petersen Way at GenCon 2018.

He wanted to keep the mechanics away from the storytelling as long as possible.

In my games, I used to do it both ways.

If I had a picture, I'd show a picture of the Mythos creature and then ask for a SAN check. The only issue is that sometimes the pictures aren't horrible enough.

Sometimes, I'd asked for the SAN check first, then described what the PCs saw because I wanted it to be a surprise. Also it got the book keeping out of the way because if I describe the horrible thing first, the Players always want to act right away before I can ask for the SAN check, and sometimes I'd forget and the Players would conveniently forget too.

But after Sandy's lecture, I started to pay more attention to this when other GMs ran a game and I came to the conclusion that Sandy's way is best.

This was how I came to this conclusion:

When you ask for a SAN check before describing the horribleness, the games comes to a complete stop as everyone rolls dice and if someone loses 5 SAN or 20%, then more dice rolls. This may take several minutes and that pulls everyone out of the narrative. Then when you're ready to describe the great horribleness, the atmosphere at the table has been destroyed and I argue less effective than if you had completed describing the great horribleness and then asked for the SAN check.

If I can describe something creepy and horrible enough, I can sometimes get a Player to voluntarily ask for a SAN check. This is great because it makes me feel like I did my job right. And if there was supposed to be a SAN check, then I'd say to the rest of the Players, "You should all make SAN checks too."

If no one asks for a voluntary SAN check, then I'd say, "Everyone make a SAN check."

Also as you finish describing the great horribleness, the Players probably have already decided their plan of action (most likely running for their lives), so when you then have them roll the SAN check and those that get A Bout of Madness will have a leg up on how they would want to react (if you gave them a choice).

So for better flow and heightened frights, I now go with describe first, SAN check second.

---

Here's another opinion with a slightly different take. Using more of a sandwich approach.

1. Hint at the horribleness with some vague description.

You see something moving in the dark, hidden behind some hanging chains and machinery.

2. Ask for SAN check.

PC loses 5 SAN and has a bout of madness. He has an involuntary reaction and freezes.

3. Complete the description based on the amount of SAN lost.

Its dripping fangs open up impossibly big as venom drips from its sharp teeth! You cannot move, frozen in fear as thoughts of its mouth snapping shut on your head, no around your neck, makes you unable to swallow. You've cut the palms of your hands with the nails of your clenched fists. How is that possible? You've dropped your shotgun. You don't remember doing this, but you must have. It's there on the ground only a few feet away, but impossibly far away.