objc to Swift : understand the syntax

I am trying to understand this objc code, so I'd like to get the Swift code conversion:

- (void)transmitMultipleCommands:(NSArray<LTOBD2Command*>*)commands completionHandler:(nullable LTOBD2MultipleCommandsResponseHandler)handler {
    if ( !commands.count )
    {
        return;
    }

    [commands enumerateObjectsUsingBlock:^(LTOBD2Command * _Nonnull command, NSUInteger idx, BOOL * _Nonnull stop) {
        LTOBD2CommandResponseHandler commandHandler = ( idx < commands.count - 1 ) ? nil : ^(LTOBD2Command* command ){
            handler( commands );
        };
        [self transmitCommand:command responseHandler:commandHandler];
    }];

}

I don't understand this syntax:

LTOBD2CommandResponseHandler commandHandler = ( idx < commands.count - 1 ) ? nil : ^(LTOBD2Command* command ){
        handler( commands );
    };

What is the goal ?

Thank you in advance for your help.