HTTP/2 Server Push allows an HTTP/2-compliant server to send resources to a HTTP/2-compliant client before the client requests them. It is, for the most part, a performance technique that can be helpful in loading resources preemptively. Read more on Wikipedia
If you enable this feature Swift Performance will generate special rules for htaccess, and also will send Server Push headers if the request is served via PHP.
The pushed files will get a desktop- prefix for desktop version and mobile- prefix for mobile version (if Separate Mobile Device Cache is enabled)
You can add a version prefix if you define SWIFT_PERFORMANCE_PUSH_PREFIX constant in wp-config.php
filenames will be 2-desktop-critical.css, 2-mobile-critical.css, and so on. It can be useful if you make changes in CSS/JS to bypass browser cache.
You can fully control filenames with
swift_performance_critical_css_filename, swift_performance_css_filename and swift_performance_js_filename filters.