Units that lose a close combat (ie they suffer more wounds than they inflict) must test against their Leadership to hold their ground. If they fail, the losers must fall back. The Leadership test is modified by the following factors:
-1 If the unit is below 50% starting strength.
-1 If the losing side is outnumbered by its opponents*.
-2 If the losing side is outnumbered 2:1 or more.*
-3 If the losing side is outnumbered 3:1 or more.*
-4 If the losing side is outnumbered 4:1 or more.*
* Only apply the highest of these modifiers.
EXAMPLE: A Dark Eldar warrior squad has lost an assault. They would normally require an 8 or less to pass the Leadership test, but are below half strength (-1) and outnumbered 2:1 (-2), which means they require 8-3=5 or less. They roll a 6 and fail.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): Outnumbering is calculated on a unit-by-unit basis, so even if one model from a unit is fighting, he'll get the benefit of the rest of his unit backing him up when it comes to intimidating the opposition. Likewise, if the models actually fighting are killed but there are a million more of them in the unit, they'll still outnumber the unit they were fighting.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): When determining if a side is outnumbered in a multi-unit combat, work it out as whole sides, just like totalling up wounds inflicted in multiple combats.