Your question is wrong, so to speak. What's happening here isn't "ampersand and colon", it's "ampersand and object". The colon in this case is for the symbol. So, there's & and there's :foo.
The & calls to_proc on the object, and passes it as a block to the method. In Ruby, to_proc is implemented on Symbol, so that these two calls are equivalent:
something {|i| i.foo }
something(&:foo)
So, to sum up: & calls to_proc on the object and passes it as a block to the method, and Ruby implements to_proc on Symbol.
