What's this weird thing I can do with my hand?

[guysensei1]

Here's a video: https://www.youtube.com/watch?v=iHtIPbYK_EI

I have been able to do this for as long as I can remember, and only on my right hand.
The action that I do to trigger this thing involves pressing my 4th finger onto my palm.
Doing this doesn't hurt even if done continuously. Any ideas what this is?

 

[Greg Bernhardt]

Looks to me like your moving the tendons from side to side over your knuckles?

 

[bobze]

Greg is correct, you are moving your extensor tendon over your MCP joint. As you can see in Greg's picture the sheath over the 5th and 4th digit extensor tendon isn't as developed as the 2nd and 3rd digits. Yours seems to be loose enough you can move it back and fourth.