What is the difference between optirun and primusrun (bumblebee)

I have installed the Lightworks video editor on Debian Jessie. For best performance it needs to run on a discrete video card with the proprietary driver. A Nvidia GTX 860M in my case. I have installed Bumblebee to switch between video cards as needed. With optirun or primusrun it is possible to run an application using the Nvidia card. When I use optirun for Lightworks it crashes after startup. When I use primusrun it doesn't and performance is okay. Why is that? What is the difference between the two? This question has been asked before , but remains unanswered. This answer on a different question does allude to a difference, but doesn't explain it.