Put the "optional" tasks in a sequence container. Then have the precedence constraint from task 15 (or whereever you're checking your conditional value) go to the sequence container using "Success and Expression" as the conditions of the constraint. To edit the constraint, right click the line leading from the task to the container and click "Edit". There is where you set the values and have the conditional verified.

Pictures attached. First one is the right click menu. Second is the window with the editor.

EDIT: Something I forgot to mention. You can have multiple precedence constraints going from task 15 to other tasks. You can also have the opposite check for task 15 to task 19 (or 20), i.e. Value is False. And to make sure task 19 or 20 runs after the sequence container (if Value is True), you add a constraint leading from the container to task 19 or 20. It'll look like a triangle with the arrows going from 15 to 20 and 15 to container and container to 20. 20 will NOT have an arrow going back to either 15 or the container.

Make sense?