Not sure if your 6000 is the same but this is a pic of the release lever(another one right side of motor) on my ZX5400. Right now the levers are in the position so you can roll the mower around. Hope this helps.
If you do not have an owners manual, go to TORO's web page and download one . They are free.
Older ZTR's had a bypass valve with a lever or rod on them.
Newer models just have a nut, one on each motor or pump so there will be 2 on your mower.
Usually it has a funny looking head so when you look down there will be dozens of plain hex headed bolts then one with slots or dimples, this is the bypass valve.