An Executive Summary Tree is not another type of tree in the Thinking Processes tool box, but a concise, condensed form of a Current Reality Tree or Future Reality Tree for presentation to executives.
Bill Dettmer got used to make short, concise briefings to general officers in his career in the military. Their civilian counterparts, high-ranking executives and decision makers have no more time and patience, thus need brief presentations too.
Two trees in Thinking Process* are especially important: the Current Reality Tree and the Future Reality Tree. The first describes the links from Undesirable Effects (UDEs) or actual problems to their few root causes. The second depicts the Desirable Effects (DEs) and necessary injections to get there in future.
If the presentation is about the actual state, the Executive Summary Tree will be a condensed version of the Current Reality Tree. If the presentation is about where to go or what to change to, it is the Future Reality Tree which will be presented in this concise way.
The how-to is described by Bill Dettmer himself in this video.