rkzenrage

Shaking them "bruises" the vermouth.
Though, I used to have a method where I would spritz the cold glass with vermouth and place it back in the freezer.
I would make an ice slick of vermouth that would slowly melt into the gin and olives/juice.
Then you could shake-spin the crap out of the gin and get it so cold ice crystals would form. In Sapphire, this, with the ice sheet on the glass, this was a beautiful drink.

I also made one with homemade cranberry liqueur (made from only gin, sugar, cranberries, time and much straining). Blood-red and very tasty.

Edit: and I only use, cold, Spanish giant queen olives or Texas Pete's hot pickled okra (to DIE for).
Bruising means you don't get the separation of flavors, the gin and vermouth separate on the palate. Bruised, the drink is cloudy, not only in color but in flavor as well... I would not drink a bruised martini unless it was the only thing to drink. It tasted like strange water.

The only time I have ever place the gin and vermouth in the same container before the glass was mixing in a pitcher and placing in a freezer or fridge. Usually I had a spritzer, like what hair dressers use on your hair, filled with vermouth and some juice of olive or Pete's hot okra juice to spray in my glass before placing it in a freezer.