To quote from their official site: Choose the right stick of your choice for your favorite mobile game from a 3-piece mini stick:
XBox 360 analog stick shape concave design, PS3 analog stick shape convex design and D-pad shape.
To use the mini sticks, a player simply sticks it in place and starts.
Arcade game stick compatible with mobile / iPhone, iPad and tablets
Add a real physical mini joystick to your mobile phone or tablet for enhanced gaming precision
Works with most of games
No wires or batteries required
Won't damage the screen
Easy to remove and reposition
Contact touch-screen sensitivity
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actually its not hard to have the proper material under the so called buttons which are probably more like padding rather than buttons just like the screen protectors and stylus pens. Most have material to touch the screen without causing any damage.