Good question!
Apparently your question is on backward masking, which means that the stimulus (the probe) follows the masker in time. Backward masking generally occurs at higher levels, typically the cortex. In case of visual stimuli this can be the primary visual cortex, or V1 (Mace et al. 2005). Ongoing processing of the probe is then thought to be interfered with by the subsequent presentation of the masker, as long as the masker-probe interval is not too long. The PNAS article by Covaks et al., 1995 shows this nicely at the cortical level: neurons working on the stimulus change their response patterns when a masker is presented afterwards. So the existence of connections folding back and forth onto themselves are not necessary to explain forward masking. Cortical processes are often affected by consciousness/alertness so the presence of backward masking effects can be interpreted as a marker of cortical processing/higher-level processing.
Forward masking on the other hand works at the neural level through adaptation processes. Although the effects of forward masking can be observed at the cortical level as nicely explained by Brosch & Screiner 1997, forward masking is caused by processes occurring in the periphery (cochlea or auditory nerve in hearing) and are unaffected by mental state. Hence, the presence of forward masking indicates that the neural process under investigation reflects a peripheral process that can also occur subconsciously, whereas the presence of backward masking generally point towards a higher-level neural process that is typically affected by mental state. Hope this helps.