The SW-R610 remote sprint shifter can be mounted anywhere on the drops - they allow for quick shifting the cassette while maintaining a secure grip on the handlebars.
A pair of shifters is rather light - they weigh 5.8g. The right shifter shifts down the cassette while the left one shifts up the cassette.
Note that these shifters won't function on their own. They require the use of a dedicated sprint shifter port, only found on these components: