I see a swing bucket rotor for PCR trips and don't know what step is using? What difference if I use fixed - angle? In addition, Which step I can use swing bucket instead fix - angle in molecular biology (Example: extraction, purification,..) to improve centrifugation efficiency?
For a PCR preparation, swing bucket or fixed angle does not make any differences. The only thing you want is that everything is soluble, in the same volume, and homogeneously mixed.
The main difference between swing bucket and fixed angle is how the pellet will be formed in the tube. Depending on the type of extraction or purification you are doing it would be advantageous to have a large pellet on the side (fixed angle) or at the bottom (swing bucket).
This article explains it in much more details if you are interested: https://handling-solutions.eppendorf.com/sample-handling/centrifugation/this-and-that/detailview/news/when-to-use-a-fixed-angle-rotor-or-a-swing-bucket-rotor/