Steve is fortunate enough to have an amazing 'device' through which he re-lives the days of having two legs. The c-leg is, according to its manufacturer Otto Bock, a "fully microprocessor-controlled knee joint that allows for previously unachieved level of safety and dynamics. It forces a natural gait pattern for safe, smooth walking at different speeds and on all surfaces."
He charges it every night before bed, just like a cell phone. When he see the prosthetist, they connect the leg to a computer and program the leg to precisely how steve walk, stands and shift weight.
For detailed info on the c-leg, check out the manufacturer, Otto Bock.