We recently worked a project for client and one of the requirements was a parallex banner feature.  This feature exists for Squarespace 7.0, but at time of putting this article together, not such feature currently exists for Squarespace 7.1.  However, a work around currently available thanks to Chris over at Schwartz-Edmisten Web Design  

Below is the code, in your Squarespace site dashboard, then navigate to ‘Settings > Advanced > Code Injection’ and paste the following code into the Footer section of the code injection area:

Note: it does not work on mobile, but work on desktop. It also does not work for video sections.