If increased output from comes more off-the-book hours, then unit values are not reduced: each commodity still requires the same labor time to reproduce it. Without reduction in unit values the wage as value of labor power cannot fall; hence, the possibilities of relative surplus value are blocked. A higher rate of exploitation cannot be achieved but through an attack on the value of labor.
That is, if the greater output is coming from more hours or (what is the same thing) more intense hours (e.g., less downtime, see Slaughter and Parker), then the value of labor is falling, i.e. real wages are now less in relation to actual hours worked. This in essence represents the growing immiseration of the working class.
See Geoffrey Kay, The Economic Theory of The Working Class, MacMillan, 1979, p. 76ff.
Moreover, without the reduction in unit values that is not achieved by increasing output through longer hours, realisation difficulties mount: it becomes more difficult to realise a greater mass of use values on the basis of market expansion engendered by lower unit values.
Yours, Rakesh
ps1. Roger, just saw your message. will reply later. ps2. Doug, profit rate does not have to fall to zero to choke accumulation. See Grossmann.