Figure 2 shows the available transitions in the top-down algorithm.
the Shift transition moves the first (leftmost) word in the buffer to the stack; the Non-Terminal-X transition pushes onto the stack the non-terminal node X that should be on top of a coming constituent, and the Reduce transition pops the topmost stack nodes until the first non-terminal node appears (which is also popped) and combines them into a constituent with this non-terminal node as their parent, pushing this new constituent into the stack.