The way it was originally designed, there is an angled metal plate behind it which is supposed to deflect an incoming ball down into the saucer, and also deflect the ball back towards the player when it is ejected.
Unfortunately, the metal plate has never worked very well for the majority of IJ owners. Typically, many shots directly into the mode hole will bounce right back out, meaning it can be difficult to start a mode, or complete any reward that requires the mode saucer. This is probably the most important shot in the game, and it's always been frustrating.
Since the dawn of time, IJ owners have tried to fix this problem so that the mode hole "sticks" better. From bending the original deflector, to replacing it, to removing it completely, there is a TON of information about what people have tried. Foam, rubber, prayers...you name it.
You may have noticed a lot of talk here lately about ricochet's upcoming deflector replacement called "Lao's Antidote." This is a complete metal replacement for the existing deflector plate, honed through clever engineering and rigorous testing. Once it's out in the wild, may we never have to fuss with the "mode hole problem" ever again.