  We study the origin of the predictive skill of some methods to forecast the
strength of solar activity cycles. A simple flux transport model for the
azimuthally averaged radial magnetic field at the solar surface is used, which
contains a source term describing the emergence of new flux based on
observational sunspot data. We consider the magnetic flux diffusing over the
equator as a predictor, since this quantity is directly related to the global
dipole field from which a Babcock-Leighton dynamo generates the toroidal field
for the next activity cycle. If the source is represented schematically by a
narrow activity belt drifting with constant speed over a fixed range of
latitudes between activity minima, our predictor shows considerable predictive
skill with correlation coefficients up to 0.95 for past cycles. However, the
predictive skill is completely lost when the actually observed emergence
latitudes are used. This result originates from the fact that the precursor
amplitude is determined by the sunspot activity a few years before solar
minimum. Since stronger cycles tend to rise faster to their maximum activity
(known as the Waldmeier effect), the temporal overlapping of cycles leads to a
shift of the minimum epochs that depends on the strength of the following
cycle. This information is picked up by precursor methods and also by our flux
transport model with a schematic source. Therefore, their predictive skill does
not require a memory, i.e., a physical connection between the surface
manifestations of subsequent activity cycles.
