Glue mainly, although i think quite a lot of things have been tried in the past . ie. sandwiching in different materials
I am told that one of the best is rubber based glue, but there are lots of theories on it.
I was told that a lot of internationals were using them but have been told to stop it, so ithe trend, certainly internationally is dying out.
Most local umpires struggle to couunt to six amongst everything else so i woudl doubt that you would ever get called up on it.
The only risk would be coming a cross a bad nerd who would only spot it if they had a really good look at your bat.
I covered all the open open bits with bat tape and side stickers, but mine was more obviously a laminate as the two bits of wood are different colours.
Front face with something like 17 straight grains and edges of about 40+ millimeters.
Tis a beast! and all for £115 pounds