This strategy lets you mark a project as either parallel, meaning you can do all of the tasks anytime, or as sequential in which case you work on it step-by-step. With sequential projects all tasks depend on the one before.
After you go to ☰ → Strategies → Sequential Projects and enable it, you will get an option for your projects that lets you switch them between parallel and sequential. This option can be found in the "Edit" menu that you can access via right-click menu or hover button actions.
This strategy is used with the Backburner because it will put all of the tasks that are not the first step into the backburner.
You can open the backburner by pressing "B" on your keyboard or by clicking on the backburner icon on the top-right.
Changing the *nextStep advanced filter to *isFirstStep
When this strategy is on, the *nextStep advanced smart filter will match ALL children in parallel projects (which is the default project type). To mimic the old behavior, use *isFirstStep. You can read more about these two filters in the Advanced Smart List Filters article.