How does sender.selected = ! sender.selected toggle between selected states?

How does sender.selected = ! sender.selected toggle between selected states?

I'm very new to Objective-C so sorry if this is extremely obvious to many
of you, but I'm trying to work out how the following piece of code is
actually working:
- (IBAction)chooseColour:(UIButton *)sender {
sender.selected = !sender.isSelected;
}
Now it obviously toggles between the selected and unselected states of the
button sending the action, but what is the code 'sender.selected =
!sender.isSelected' actually saying? Is it just 'set the sender selected
property to the opposite (i.e. ! not) of the getter'? So if the getter is
'getting' the current selected value as true then it sets the selected
property as !true i.e false. Or is this a piece of convenience code that
I'm not yet privy to? Because it also seems that '!sender.isSelected'
simply means not selected as in
if (!sender.isSelected){
statement
}
i.e. do statement if the sender is not selected. This is no doubt really
obvious, just I'm a bit confused with it at the moment.
Thanks!