Here is the sample usecase to demonstrate how reentry-not-allowed and reentry-allowed properties works.

what doc says about these 2 properties :

reentry-allowed: Reentry is allowed on any view activity within the ADF bounded task flow

reentry-not-allowed: Reentry of the ADF bounded task flow is not allowed. If you specify reentry-not-allowed on a task flow definition, an end user can still click the browser back button and return to a page within the bounded task flow. However, if the user does anything on the page such as clicking a button, an exception (for example, InvalidTaskFlowReentry) is thrown indicating the bounded task flow was reentered improperly. The actual reentry condition is identified upon the submit of the reentered page.

Ingrediants :
main.jspx -> Jobs_TF -> jobs.jspx

scenario.
click RunTrx button in main.jspx navigates to jobs page by entering into Jobs taskflow.
click jobs page back button to navigate back to main.jspx, now click browser back button to navigate jobs.jspx and then click jobs page back Button to see reentry-not-allowed error message.