How does `process.nextTick` keep my stack from blowing up?

I've stubled upon a function (here on SO) which writes a file, but makes sure to not overwrite a file:

function writeFile(i){
    var i = i || 0;
    var fileName = 'a_' + i + '.jpg';
    fs.exists(fileName, function (exists) {
        if(exists){
            writeFile(++i);
        } else {
            fs.writeFile(fileName);
        }
    });
}

Now there is an interesting comment below which says:

Minor tweak: Since JavaScript doesn't optimize tail recursion away, change writefile(++i) to process.nextTick(function(i) {writefile(++i);}); that will keep your stack from blowing up if you have to go through lots of file names.

Please explain. How does process.nextTick prevent the stack from blowing up?