What causes "shutoff shudder"?

[Tab Julius <tab@penworks.com>]

Turning off my ignition, my 78B's engine will often give me a few shudders
before it decides to shut off for good.  This is an effect I remember
fondly from other older manual, probably non-fuel-injected cars (most
notably my long-gone yellow 72 beetle).  Releasing the clutch (to a gear)
causes it to shut off quicker (okay, stall).

Some basic tuning somewhere needs to be done?  Or is it just having so much
fun it doesn't want to stop?

Thanks

- Tab