This is a measure of the rate at which characters become involuntarily unplayable (through non-resurrected death, crippling, permanent incapacity, concept-destroying transformation, maiming, reduction to idiocy or insanity, going permanently MIA, left for dead as the mind-controlled thrall of an Illithid, campaign-ending TPKs, etc.)
A campaign's milliwhack (mW) rating is calculated as follows:
1. Add up the number of involuntary-unplayable events (usually character deaths, but see above).
2. Divide by the number of sessions in the campaign.
3. Divide by the average number of players per session.
4. Multiply by 1000.
I played about three years of bi-weekly, Paizo adventure path 3.5e and in that time we had two TPKs and perhaps two PC deaths in the interim. Call that 60 sessions with a party of 4-5 (so, 9 unrecoverable deaths) so that play style rates 33 milliwhacks.
11 deaths / 60 sessions / 4.5 players-per-session * 1000 = 41 mW
In four one-sessions games of four-player Fiasco, I've seen maybe two deaths (both mine). So that play style rates 125 milliwhacks.
2 deaths / 4 sessions / 4 players-per-session * 1000 = 125 mW