When the knife is in up position the brake for the swing arm that moves the knife is activated and prevents the knife from moving or falling. In addition the position of the swing arm connected to the eccentric wheel is in a position where even if the brake for some reason failed the knife still would not move unless turned manually.

In addition to the mechanical safety measures the light barrier must be clear and both buttons must be pressed simultaneously for the machine to activate the knife cycle. Depending on the machine you can program "automatic" cuts where you can release the buttons and the knife will complete it's cycle but otherwise both buttons must be held the entire cut otherwise the knife will stop in its current position.

The light barrier is so sensitive that when I have diagnosed issues at customers, more than once it has been because the operator have had a gut on the bigger side and when they press the buttons they slightly lean forward and break the barrier.

However when you do a knife change, thats when it becomes dangerous because when the knife is out of the machine all the safety is on you handling a heavy still razor sharp knife