I'm going to guess....you put the tip of your thumb on the bar and use the pad of your thumb to control the spool without digging your thumb into the reel? But the question is...what is the (silver) bar above the spool attached between the sides of the reel going across? I just noticed it when I got this other reel (the middle one) and it didn't have it...

Or is it to help against free spooling too fast and getting the line pushed back down to the spool during a cast? Sort of like a limit bar or something...I don't know...your guess is as good as mine

it's for connection of the brake side side cover , some reels designs have a different side cover release/removal , it also gives alittle strength to the reel between the side plates giving more integrity to the reel .

it has nothing to do with casting ...

How do you take the side of the reel off? I have one that has a switch and it says (open). But the others I dunno

My revo is like that.  There should be a screw on the right side of the bar, on the outside of the reel.  Loosen that up.  The screw may come out or may stay in.  My revos just come loose, but do not come out.

When the screw is loose, the left side of the reel will either come off, or will unlock so you can turn the cover and remove it.

Ya that's just a screw to lock the side plate down. Loosen it all the way and rotate the opposite side plate to remove. Sometimes the screw has a small spring so you have to pull out on it to release the plate. 

Sweet, thanks KS for the help guys! I do use it as a thumb rest though