Of course, 90% of you knows it allready. But if you aren't in that percentage and don't "waste" time reading manuals then you know now.
With pressed ALT key Burn Tool turns into "Lighten" Tool. Options for it are same as Burn Tool, so bigger % in Exposure it will lighten more place you paint with it.
Tip: don't set Exposure more that 5% (maximum 10%). Better paint twice that will be more accurate.
Usage: For quick lighten some places on image without masking and selectiong. In overall it isn't most precise tool in PS. But for quick and fast lighten it's good. Like 5cents improvment in 5 seconds. Good deal
Hope it's usefull.
P.S. I found it years ago by accident and keep using it almost everyday.