When a packet has occupied a channel no other packet can use it.
If packet becomes blocked, the occupied channel becomes idle even if other packets can make productive use of it.
Packet 1 routed through channel A and channel C.
Packet 2 wishes to go through channel B.
Assume packet 1 blocked. Therefore channel C idle, even though packet 2 could make productive use of it.
Furthermore packet 2 can then block further packets resulting in tree saturation.
Dally claims this effect limits throughput of IN to fraction (20%-50%) of network's capabilities.