What is the thing circled in red?
Pancake pneumatic piston.
Used for shifting.
http://www.vexrobotics.com/vexpro/pneumatics/217-2778.html
Pancake Cylinder.
That’s a pancake Cylinder. It’s used in 2+ speed gearboxes to shift gears on the fly.
Your link says Pancake Pneumatic Cylinder"
Brought to you by the IACNAP campaign.
“It’s a cylinder, not a piston”
The piston is the piece inside the cylinder which is pushed by the air and which in turn pushes and pulls the output shaft relative to the main cylinder body. P4s (pernickety persons’ pet peeves) aside, the term piston is commonly used for the whole thing because it won’t be confused with the shape of a pneumatic tank, compressor, motor, wheel, or shaft.
OBTW, if the one in the render above is a cylinder, we can also include **any **sort of extrusion as a cylinder by shape.
If you call it a pneumatic cylinder, it might remove some confusion
I’ve had team members come back with air tanks (more commonly aluminum than plastic). We keep the tanks and actuators in different bins.
Yup, that’s why I said “might”, rather than “will”.
I don’t know a single person in industry who would call it a piston. As mentors it’s up to us to teach proper terminology.
IndySam is correct. If we are to prepare these students for working in the industry, we should teach (use) proper terminology. Incorrect, or non-standard, terminology leads to confusion and mistakes.
It is a cylinder or pneumatic actuator.
It can be used as a piston when attached to a cam but it is not a piston.
And here I am calling them pneumatic actuators…
We have had some engineering mentors call them pistons and we correct them at the same time. These are also mentors who never work with these types of components in their day jobs.
When I went shopping on FIRST Choice for our team, I selected several pneumatic cylinders, but no pistons.
The pneumatic cylinders each contain a piston, though
And I always assumed it was a British thing. They are always screwing up the English language over there. :]
And misspelling things, too! 
Kidding, kidding… I’m Canadian. We misspell them the same way!
Using the “colour” white huh?
FTFY