The QSlide SDK allows developers to build an app with the floating window transparent underneath, so that users can view the background app. Code jockeys can also add the QSlide icon to the notification bar, allowing those using the app to quickly launch the feature.
LG has been including QSlide in many of its new models. Besides the high end LG G2 and LG G Flex models, the feature is also found in the QWERTY equipped LG Optimus F3Q.
QSlide in action (L) and the notification bar
source: LGDeveloper via AndroidCentral