Full Idea
Efficient causes comprise both the presence of stimuli which activate a quiescent individual, and the absence or removal of constraints upon an individual already in a state of activity.
Gist of Idea
Efficient causes combine stimulus to individuals, absence of contraints on activity
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 1.II.B)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.12
A Reaction
This is part of an account of causation in term of 'powers', with which I agree. Before you object, there is always going to be something about causation which is mind boggling weird, and probably leaves even God bewildered.