Effective Encumbrance (EEC) is a value that represents the effects of (the weight of) a hero's armor and the items she carries in her inventory on the various Talents. Think of it like this: Heavy armor is more cumbersome to a Burglar trying to Sneak, than to a Warrior attacking a band of raiders.
Thus, each type of armor comes with a value for Encumbrance (EC), telling you how much this armor restricts your hero in certain situations. The higher EC is, the harder actions like climbing, swimming or fighting in this type of armor get.
The EEC of the Spears Talent is: EC-3. So if our hero were wearing a long chain-mail shirt (EC 4) while fighting with a spear without any other Combat Talents, her EEC is calculated as: EEC = 4 - 3 = 1. If, however, she were wearing only ordinary street clothes (EC 1, like a Burglar's outfit), her EEC equals 1 - 3 = -2 (=0).
EEC is automatically subtracted from your hero's Talent Prowess (TP) just before rolling any Tests. A positive total EEC value results in a Difficulty Increase for these Tests, but a negative total EEC value does not result in a Difficulty Decrease! In fact, a negative EEC does not have any effects whatsoever; for all gaming purposes, EEC values below 0 are treated as EEC 0.
However, the resulting TP after subtracting the EEC and other modifications, CAN drop below zero. If that is the case, the hero needs to add this amount to all of her three subtest rolls (while still trying to beat her Attribute Values). So a negative TP always leads to Difficulty Increases on all three Tests.
Note, that weapon class modifier to encumbrance (e.g. -3 from spears as in example above) is only considered for AT and PV values. How other talents relate to encumbrance, is shown on info sheet after pressing RMB. Majority of non-combat related talents (e.g. seduce, perception) usually ignore encumbrance, but some of them (such as pickpocketing, sneaking) can take great penalty from it.
The hero's EC value (and thus the EEC derived from it) is influenced by more factors than just the armor worn. Excess weight of inventory items can increase the EC. See Encumbrance for details on this.