Dual-screen display is while you're viewing a photo or video in HTC Gallery, playing music in the HTC Music and watching movies in HTC Watch on the big screen, you can multitask to do other things such as take calls, browse the web, or send email.
Currently, Droid Incredible 4G LTE by HTC, HTC EVO 4G LTE, HTC J, HTC One S, HTC One XC, HTC One XT, HTC One XL and HTC One X have full functionality.
For HTC One V, HTC Desire V, HTC Desire VC and HTC Desire VT are compatible, but only some services are supported, e.g. HTC Gallery, HTC Music & YouTube. It is not supported to project what’s on your phone screen to your TV.