What are the braces in this makefile rule for?

I am reading a makefile for a Qt-created project that has the following:

{backend}.cpp{release\}.obj::
    $(CXX) -c $(CXXFLAGS) $(INCPATH) -Forelease\ @<<
    $<  
<<

(above code is using \t for recipe and is as written in makefile)

Both the rule and the recipe confuse me.

I'll start with {backend} in the rule. Obviously the same confusion for {release} as well. I assume this is a reference to a particular sub-directory named backend. I guess that ..\backend\release\bar.obj would be found as a legitimate target? But what part of make says this is legitimate syntax and what exactly happens here?

FWIW: This is in a section commented as: ##### implicit rules. Version: GNU Make 4.2.1 Built for x86_64-unknown-cygwin

Bonus points:

Explain the use of @<< and << in the recipe... (Yes, I'm lacking in bash shell finesse...). Is this referencing the first prerequisite with $< and silently redirecting it? Why isn't it $$<?

Thanks.

That is an NMAKE batch-mode rule

https://docs.microsoft.com/en-us/cpp/build/batch-mode-rules?view=vs-2017

The equivalent GNU Make rule would be something like

backend/%.obj: release/%.cpp:

With the difference that, as the name suggests, these rules will invoke their recipes only once for all valid targets and expect the rule to create all of the targets in a single pass with the $< macro.

The << syntax is NMAKE's inline file feature

https://docs.microsoft.com/en-us/cpp/build/inline-files-in-a-makefile?view=vs-2017

This expands and captures everything between the angle brackets and saves it to a file, in this case a temporary file as no filename is specified after the brackets. The file is then passed to the compiler as a response file on the first line of the recipe through the @ option.