Above circled in blue is a weight hole and for this particular ball it was going down the lane to far before it wanted to make its motion. Once I added a weight hole it made the ball motion start sooner and was more angular down lane. This ball would stay in my bag frequently and I would rarely take it out just because I couldn’t ever throw it. Once I added the weight hole I really fell in love with the ball and started to like the motion on it.
Weight holes are all based on how a person throws the ball and for me where I have it like I said makes it start a little sooner and has more angle. For some people this would make the ball be calmer on the back end and not have a lot of motion on the back. Weight holes can either make a ball start sooner and have more angle or even make the ball be smoother and have a more consistent reaction.
The other larger hole is called a switch grip, this allows me to remove my thumb from the ball and use it in a different ball. Most pros and amateurs use this system because we want the same feel in every single ball. We get fit for a reason and want to have consistent fits in every ball. Also people with swelling issues like myself have different sized thumbs. Each thumb is the same in the sense of how your thumb fits in but people will have different sizes for when their thumbs swells up. Example would be that I have 4 thumbs that I can interchange at anytime each thumb is fit the same just different sizes. I have a small, medium, large, XL so when my thumb is bigger I use the XL when my thumb is smaller I go smaller. Below is what the mechanism looks like and all you do is screw in the thumb and you can switch it between any balls you would like.