To be honest, nothing.
You can't do anything about the flexibility of a storebought rubiks 3x3 or any rubiks 4x4. You can lubricate with silicone spray (not wd40 or vaselline) to make them quicker, but nothing can change the flexibility.
The cores of a storebought 3x3 attach to the centres with rivits rather than screws, so they can't be adjusted. Then they glue down the caps to prevent you getting access to the rivits.
Btw, this topic has been moved from the website board to one more appropriate.
Please take off topic discussion off this forum. Thanks.