Nikon F4s shutter

I would like to test the shutter speeds on my F4s. I cannot get the shutter to fire with the back open. I'm sure there's a release button some where on the open back, but I can't seem to locate it. Any help?

Open the back, look at the groove where the seal is supposed to be and right underneath the rewind crank. There is a little silver button there. There is another one on the other side but you only need to push the one on the rewind crank side and the shutter works.
At a swap meet there was a guide who tested shutter speed and he used break a piece of a toothpick and jam it in there.

From page 110 of the instruction manual:

Download the instruction manual here: https://drive.google.com/file/d/1Zccpg3z3UsI5H7mKxliicZOCblbAxMr4/view?usp=sharing

I wonder why?

Pieter12 said:

I wonder why?

Click to expand...

But I can fire my F4 shutter by pressing that little button I told you about.

I wonder why Nikon warns against using shutter speeds slower than 1/250? Possible damage to the curtains?