As with anything, there is a right way and a wrong way

Checking chain tension on a motorcycle, before you even start remember, when working with machinery there is a right way and a rong wrong way. 

Always think safety, power off before you do anything.  

What you are looking for is play, a chain requires play or it wears. Too much play cause slap and damage.

In my youth I was working on my first motorcycle, a Yamaha 80. I was removing the chain which had come off the sprocket and wedged between the drive sprocket and the housing, foolishly I left the bike running when I did it. To access the area the shifter and side cover had to be removed, I yanked up on the chain to free it and hit the shift rod. the bike was on a center-stand, hitting the rod put the bike in gear and my fingers were pinched between the sprocket and chain. Luckily the chain was not on the rear sprocket so there was plenty of play. After my trapped fingers made the rotation from the top of the sprocket till the chain and fingers came out the bottom and allowed me my had back, I looked at them expecting to see mutilation, luckily there was to tension and the fingers were just chewed up a bit. 

Not everyone is so lucky. maybe it was the Lord above watching over fools and children, or foolish children but I came out alright.

Not everyone is so lucky.

Always take the time to think of what could happen if you don't take all your precautions. 

I do not know the circumstances which caused the below hand damage, but this is what happens when your chain works your flesh.