Advanced user mode can be enabled from the Settings tab in uBlock Origin (uBO)'s dashboard. Enable at your own risk.

Read the docs

Advanced users are expected to read the documentation. This is very important. If you use advanced features without fully understanding them, uBO: might probably will ... behave in ways unexpected to you, and most likely you will end up being LESS protected than you would otherwise be with just default settings. DO NOT UNDERESTIMATE THIS WARNING.

Differences

Advanced settings

You will be given the ability to access the advanced settings. Some of these settings are experimental, they may become standard settings or disappear in the future.

Dynamic filtering

Dynamic filtering will become available to advanced users.

Novice users could easily mess up uBO's filtering through dynamic filtering, thus it is not available by default.

On the other hand, if you are familiar with RequestPolicy, then you should have no problem dealing with dynamic filtering.

Important note: Dynamic filtering engine is completely turned off when you un-check the setting "I am an advanced user". Your dynamic filtering rules are kept intact though, in case you re-enable advanced user mode again.