Here's how to enable the new IE12 Trident mode in Internet Explorer on Windows 10

By default it is set to 'Automatic' which lets IE decide.

Microsoft has moved ahead with Internet Explorer's new forked rendering engine, including it in the latest 'official' build of Windows 10. A new flag in the browser allows users to control when it is used. The new engine is Microsoft's attempt to break away from legacy features in Internet Explorer.

Microsoft earlier decided to 'fork' the Trident rendering engine into two parts. If a website calls for compatibility mode, then the older and more resource intensive Trident engine from IE11 will display the site, otherwise, the more lightweight and updated IE12 Trident engine called "Edge 12.0" will handle it instead.

By default, Internet Explorer will decide which engine is used, however, that can be changed on a new "flags" page included in IE12, called "Experimental Features", which is located by typing "about:flags" into the address bar. Other versions including IE11 on Windows 8.1, do not have this page.

On the new configuration page, Microsoft refers to the new rending engine as "Experimental Web Platform Features" and gives you three options, including "automatic", "enabled", and "disabled".

The new engine and flag appears in Windows 10 Technical Preview build 9879.

Source: McAkins