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When a unit in combat is hit, it runs a risk of suffering a devastating critical hit. In short, the risk to take a critical hit is normally 1/36, or 3/36 when above 50 fatigue, or 6/36 when immobilised. A critical hit halves protection.
Note that critical hits are a separate mechanic from head hits.
On a hit, first armor-piercing effects (AP weapon, AN weapon, piercing weapon) are applied to the targeted unit's protection. AP and piercing are done separately, so they are multiplicative (40% when stacked).
When a critical hit occurs, the unit's current protection (after AP stuff above) is divided by 2 (rounded down, all divisions are btw). Afterward, the protection DRN is added to that protection to get the protection used against damage.
Note that it is the same DRN roll that is used for both reducing damage and deciding whether a critical hit happened, which leads to critical hits being even more devastating.
This disassembly snippet was posted by iRFNA's in 2015:
v128 = opendice(6, 2); protdrn = v128; if ( hascurseluck > 0 && randnum(100) < 50 ) // curse luck is internal effect 255 protdrn -= openroll(6); if ( *(__int16 *)((char *)fatigue + hitunitoffset3) >= 50 ) --protdrn; posprotdrn = ((protdrn <= 0) - 1) & protdrn; // set negative prot drn to 0 if ( posprotdrn > 2 && (immobileunit(hitunitid2) || !immobilized(hitunitid2) || posprotdrn > 4) )// mobile unit has 1/36 chance to crit (protdrn <= 2) // immobilized unit has 1/6 chance to crit (protdrn <= 4) // this is ignoring the two protdrn alterations above protaftercrit = prot; else protaftercrit = prot / 2; isprotneg = posprotdrn + protaftercrit < 0; adjprot = posprotdrn + protaftercrit; prot = adjprot & (isprotneg - 1); //set negative prot to 0