Of all the hand and weapons forms in the Wing Chun system, Fok Sao only appears in Shil Lim Tao form, the first form. Fok Sao is not a technique used against an attack (this is open for discussion). It is a training aid in a game called Chi Sao or sticky hands drill. The concept is to develop sensitivity so that an economical response to your partner's energy can be applied. Chi Sao drills can be applied in both single and double hand. The difficulty with such a game is starting in an equal position with neither player having an advantage over the other. There is only one central line, so if one player occupies it how can the other? Fok Sao allows both partners to occupy the same line at the same time, with no openings for either.

In
this example two partners face each other. Partner on the left directs Tan Sao
towards opponents face, Partner on the right counters energy with Fok Sao. The
energy of both partners is not tense or hard and therefor counters each other.
If one partner shifts focus in a non threatening manner (up, down, left or right)
the other can take advantage.
How Fok Sao Counters Tan Sao.
Obviously the energy direction is important, but the position of the elbow and
fingers are the key to Fok Sao. By keeping the elbow in and fingers twisting
down, it gently locks the Tan Sao giving it an opening off to the side if the
Tan Sao insists on moving forward.

Fok Sao should not simply rest on the partners Tan Sao, as this would be a non threat and create an opportunity for the opponent.
Summary
If you use different energy, or have a difference of opinion to Fok Sao applications
please speak up.
One comment has come up that Fok Sao is also used in the Wooden Dummy as well as Bil Jee form, any thoughts on that?