Left skin with a matte finish--no shine/oily look Must be applied with a proper powder brush to avoid making a mess. To me, an actual brush works much better than a powder puff. In my opinion, this powder is a must if you use Lancome's Photogenic liquid foundation on a daily basis (which is a great product). Since the Photogenic line is made to master/reflect light, it works wonders WITHOUT the powder for special occasions where you will be having lots of photos taken or photoshoots (such as in my case), and want skin to look fresh and natural. However, there is a con with the "light mastering" formula--since it reflects light (like camera flashes), it makes areas of skin look shiny when not in front of the camera, especially in the sun. The loose powder does a great job of masking the shine and leaving skin with a beautiful, matte finish.
I hope that didn't sound too confusing. My final word: the Photogenic line of foundation is great, especially for when you're in front of the camera--do not use the powder in these situations. However, if you use Photogenic foundation on a day-to-day basis, the powder is a great tool to help mask shine.
(I have never used the powder with foundations other than Lancome's liquid Photogenic, but assume it works in the same ways)