The correction argument is an optional Number that specifies the time correction factor to apply to the update. This can help improve the accuracy of the simulation in cases where delta is changing between updates. The value of correction is defined as delta / lastDelta, i.e. the percentage change of delta over the last step. Therefore the value is always 1 (no correction) when delta is constant (or when no correction is desired, which is the default). See the paper on Time Corrected Verlet for more information.