Secret Nailing is a clever method of fixing a wood floor into a timber subfloor so that the nails are not visible once the floor is installed.
The best way to do this is with a Portanailer gun, which clamps the boards together at the right tension and protects the tongue from damage as it nails. It also uses specially designed nails that don't work loose over time.
Portanilers are available to hire from Broadleaf showrooms and good tool hire stores.
To fix to a timber subfloor which is less than 25mm thick, and has services or concrete beneath, you will need to use 38mm portanails to avoid exceeding the depth of the subfloor and possibly damaging pipes or wires. Otherwise use standard 50mm portanails.