We present strong evidence observed in a YBCO superconducting tube that helical flux lines can concurrently enter and leave across the inner surface of the wall of a hollow cylinder of sintered high-T-c superconductors at 77 K, and hence cut and traverse each other. The evidence for flux line cutting and cross-flow is examined in the perspective of basic concepts. The traffic of the flux lines is also described mathematically. The penetration fields across the wall and into the grains of the sintered ceramic tube are seen to correlate with salient features (peaks and valleys) of the flux line cutting and cross-flow phenomena. This enables us to claim that we are witnessing flux line cutting and cross-flow in the weak links as well as in the interior of the grains in the YBCO tube. The Clem/Perez-Gonzalez phenomenological theory is exploited in a simplified framework to describe the crucial features of the data semi-quantitatively. This analysis confirms the above conclusions and provides estimates of j(cparallel tom) and j(cparallel tog), the critical current densities for intergranular and intragranular flux line cutting and their dependence on the magnetic flux density.