In Objective-C, what does it mean to assign a weak to a strong within a block?

In Objective-C, what does it mean to assign a weak to a strong within a block? What's happening behind the scene?

e.g.

__weak __typeof(self) wself = self;

void (^cmd)() = ^(){
    __strong __typeof(self) sself = wself;
    if (!sself) return;
    ...
};