Allied Telesis AT-S39 - Manual (Page 270)

Section III: Web Browser Management
270
When a twisted pair port on the switch operating in half-duplex
mode needs to stop an end node from transmitting data, it forces
a collision. A collision on an Ethernet network occurs when two
end nodes attempt to transmit data using the same data link at
the same time. A collision causes the end nodes to stop sending
data. This is called backpressure.
When a switch port needs to stop a half-duplex end node from
transmitting data, it forces a collision on the data link, which stops
the end node. Once the port is ready to receive data again, it stops
forcing collisions.
The default setting for backpressure on a switch port is disabled.
7. Once you have made the desired changes, click Apply.
The switch immediately activates the parameter changes on the
port.