The Instant Clones feature is functionality built into the vSphere platform rather than a specific Horizon feature, and was made available from the vSphere 6.0 U1 release when it became a supported feature as part of Horizon.
It uses the VMware VM Fork technology to very quickly provision virtual desktop machines. An instant clone is created from an already powered on and running virtual desktop machine, called the parent virtual desktop machine, which is quiesced before the instant clone is created. This is what makes instant clones quicker to provision than linked clones and View Composer.
The instant clone shares its memory and its disk with the parent virtual desktop machine for read operations and is created immediately, and in an already powered-on state, unlike with View Composer-based linked clones that must power on as part of the creation process. As well...