Backtracking Sir Keir occasionally delivers step change? (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

RIS (SIR; “backtracking” indicates reversal) + <k> E <i> R (“occasionally” means alternate letters only are used); as the upright portion of a step, a riser could be described as something that “delivers (a) step change”