What does this statement in the shop manual mean about bleeding brakes?

[davepl]

According to the 72 manual, it says:

"NOTE : The combination valve, located near the master cylinder, must be held in the open position while bleeding. This can be accomplished by
installing Tool J-23709 with the open slot under the mounting bolt and pushing in on the pin in the end of the valve. Be sure to re-
torque the mounting bolt after removing the tool"

So what does it mean to "hold open" the combination valve? Why is it required?

Thanks,
Dave

PS: Please don't say it's not required unless you know why they thought it was required...