End plate effect is about making the outer end of a foil (wing) more efficient. There are two parts of this. End plates are typically associated with keels. First of all a keel works like a wing with the difference in pressure between one side of the keel and the other creating lift. The end plate caps the bottom of the keel directing water aft.
In a heeled boat, the pressure differntial at the bottom of the keel is reduced by water slipping off the tip (bottom) of the keel. This reduced pressure makes a proportion of the lower keel area less effective. By adding a plate at the end of the keel the pressure cannot 'leak' as much.
Well.... Almost. As Jeff points out, there is a difference in the pressure in the water flow around a keel, similar to that of a wing/a foil, wherein the fluid flow across the top of the wing/foil is faster than the flow across the bottom. As the speed of the flow increases, the pressure in the flow decreases (so long as the flow remains "attached", i.e. follows the curvature of the wing/foil), hence the pressure on the "top" is lesser than the pressure on the bottom of the wing/foil creating "lift". This pressure differential remains consistent (although not necessarily constant) over the entire length the wing/foil. At the "tip", particularly of a "square ended" foil, the fluid--air/water--attempts to flow around the tip to the low pressure zone on the opposite side of the foil in a path that is "normal" to the direction of the flow. (This normal flow that curves around the end of the tip is what gives rise to the vorticies seen streaming off the tip of the foil in wind-tunnel and tank testing.) To the extent that the flow around the tip can reach the opposite side of the foil, the pressure differential there is diminished or eliminated entirely, (aka "stalling") and the "lift" of the foil in that area reduced or eliminated. The addition of an end plate, blocks this transverse flow to some extent increasing the efficiency of the foil. Such end-plat treatment can be seen on some of the higher performance jets with vertical foils on their wing tips. Other approaches to this problem include curving the back edge of the foil, and to a lesser extent the leading edge, to a point at the tip as one sees, for example, on the elliptical shaped trailing edges of high performance aircraft propellers and, in yachts, rudders.
Whereas the "top" of a keel foil varies, depending upon the tack one is on, if one is attempting to use "end plate" effect on the keel to improve efficiency, the end-plate must be symmetric, extending outward on both sides of the keel (which also concentrates weight at the tip of the keel improving righting moment). The search for "end plate effect" also extends to sails in some cases, accounting for the evolution of "deck sweepers" or head sails that actually reach and "sweep" across fore-decks as the yacht is tacked (although the frequency with which these sails also scooped up water in rough head-seas limited their effectiveness and eventually put paid to their wide spread use).
It is an interesting but rather esoteric subject and whether it measurably improves performance aboard a yacht really is debatable. For more, Google "wing tip vorticies" and related terms.