A phototeck showed me this earlier this year and it definately makes a difference to the base fog as for focus simply expose a sheet of paper for 1 or 2 minutes develop and fix ,dry etc for the size prints you are using eg 10x8
11x14 to focus turn the paper over and focus on the white side when final exposure is ready turn the paper over and the black side up it works and is what i call a 10% er