How to negative weight paint 3ds max

outer

Hello again, Ive been told painting in negative weights, what I Imagined was essentially the erase weight tool was simply a case of holding down alt when you painted, but as the pic below demonstrates, alt dosnt actually do anything, except for helping me rotate around the scene. It was still painting the weights as it normally would.



Im doing something wrong, I must be as 'hold alt and paint' dosnt leave much room for confusion. Any ideas as to why holding alt isnt working for me?