This will leave the parser in a configuration whose value of j is r, and where stack elements with left span index ≤ l (apart from those referencing the new non-terminal and its leftmost child) have not changed.
constituents of T are still individually reachable in this configuration, as their left span index is either ≥ r (and then they meet the second reachability condition) or ≤ l (and then they meet the third), so T is reachable from c. Finally, if the third condition holds, then we can create (X, l, r) by applying r − j Shift transitions followed by a sequence of Reduce transitions stopping when we obtain (X, l, r) on the stack (this will always happen after a finite number of such transitions, as the reachability condition guarantees that l is the left span index of some constituent already on the stack, and that (X, l) is on the stack).