v The goal of the link lengthening in SWMM 5 it to meet the CFL time step condition for the full link depth and full link velocity at the chosen lengthening time step. If the link does not meet the CFL condition then this means the time step needed is smaller than your selected lengthening time step. SWMM 5 will make an hydraulically equivalent longer link with a smaller roughness but the same full flow velocity as the shorter link.
v If you are running a simulation in which all of the pipes are exactly full – no surcharge in any pipe – and the variable time step then there would be no need for SWMM 5 to use anything other than the minimum of the routing or lengthening time step. However, since most real networks have a mixture of partial flow, surcharged flow and pressure flow, the actual time step depth, velocity/Froude Number is different than the assumed full depth and full flow velocity. For example, the depth can be higher at one end of the pipe and the velocity higher than full flow velocity due to the water surface slope being higher than the bed slope. The only way SWMM 5 can now satisfy the CFL time step condition since the modified length is fixed is to lower the variable time step.