When you get to the end of the beer the shot glass lifts and the whiskey is released as a chaser.
I should point out that in these days of discernment...
I tried this with a US pint glass and a similar 30ml shot glass to the one in the picture, but the shot was tipping over and releasing whiskey into the drink after getting 1/4 of the way through the beer (tasty combination by the way). Should it only tip when you reach the end of the beer? Or should the whole thing be downed as quickly as possible?
It will tip towards the end of the beer. As I remember, it is crucial to brim-fill the shot glass and then place the beer glass upside down over it so that there is no air at all between the contents of the shot glass and the base of the beer glass.