Shimano are bringing out a new satellite shifter for their Dura-Ace Di2 electronic groupset. While the existing SW-7970 unit is designed to let you change gear when riding on the tops, the new SW-7972 has been developed for rear shifting while in the drops.
It's aimed at sprinters, who need to keep a firm grip on the bar and don't want to move to the hoods to change gear. There's a right-hand shifter for shifting down and a left-hand shifter for shifting up. They plug into Shimano's ST-7970 Dual Control levers. The system will hit shops in February 2011. RRP TBC.