Can anyone please tell me what he is doing? How did he get that cross margin? (white dotted lines) and how can he select all the vertices of one circular line before bisecting them?
1 In Edit Mode, press A to select everything.
2 Go to object menu Mesh > Bisect ( or Shift+Spacebar+8 ) or long click LMB
3 Click on the far left side of object and drag to the right side and press Ctrl to strighten line.
4 Release LMB to confirm bisection.