cpower is used to create a conic model of rational powers.
y = cpower(x,p);
Plot a TV-screen!
x = sdpvar(2,1); plot(sum(cpower(x,4)) <= 1);
The operator is implemented using a graph representation based on a second-order cone models, and can thus only be used in scenarios where YALMIP can propagate convexity in a conic form.
In nonconvex scenarios, or when you do not want to generate a conic mode, simply use standard power, x.^p.