Here's what the current SOGC (Society of Obstetricians and Gynecologists of Canada) guidelines state:
"After the breech crowns, fetal expulsion is invariably accompanied by cord compression and fetal bradycardia. The normally grown fetus enters this phase well oxygenated without acidemia. It may tolerate a number of minutes of delay with extrinsic cord compression, resulting in a respiratory acidosis, easily reversed once ventilation is established. A growth-restricted fetus, however, has a high likelihood of metabolic acidemia in labour due to pre-existing compromise in placental function, which reduces its tolerance to cord compression during expulsion. Therefore, fetal growth restriction is a contraindication to labour.
Significant cord compression beyond several minutes will eventually lead to severe acidosis even in a normal fetus, and prevention and treatment of expulsive delay are critical components of delivery technique."
As the PP described above, the cord becomes compressed between the fetal head and the cervix. You may see some claims that if the cord is pulsing, then all is fine and it is not compressed. This is not true. The cord pulses because of the baby's heartbeat and the flow from the baby's body. All you can tell by the pulses is that the baby's blood flow is affected the visible part of the cord. You can't tell how significant the compression is and how much blood flow to baby is occurring.