Act is one of those multi- meaning words that infest the English language. Words apparently created to trap and confuse the learner and the unwary. In our context it means to execute a decision: to do.
And if there is anything that separates the successful from the failures it is usually the execute phase of the project. This is the area where expectations meet results and the encounter is often unpleasant.
It is also an area of great political risk. On many projects the initial phase of project evaluation and selection is usually a battle of ideas and concepts. Of contending visions and aspirations. In many instances it is a conflict of estimates and benefits, project A against project B. nothing is concrete, all is conjecture.
Not so in the Act phase. Now we have a schedule and a budget to be measured against. And just because the decision has been taken to proceed it is never an irrevocable decision. Many projects are cancelled before they are completed, according to many experts not enough of them suffer this fate, and therefore this is when you enter a new iteration of the OODA loop.
You have to be continually observing what is happening and orienting towards the changing situation. You must move into the observe phase of the new loop cycle from the first day of your project's execution. Remember it is never too soon to start failing.