When making a post request to Stripe's REST api to create a charge, we needed to return information in the balance_transaction object. Stripe offers the ability to expand properties so you don't have to make an additional request to their api for more information, which is cool. To expand a property you add it to the request parameter expand[]. But when making a post request (e.g. creating a charge), I had to wrap it in square brackets like so: [expand][]. Here is a quick example written in C#.

var collection = new NameValueCollection() {
    {"amount", (100).ToString("F0")},
    {"currency", "GBP"},
    {"source", token}, // using stripe connect to split a charge 
    {"destination", destination}, // using stripe connect to split a charge
    {"application_fee", (100).ToString("F0")},
    {"[expand][]", "balance_transaction" }
};

var client = new WebClient {
    Credentials = new NetworkCredential("secretKeyHere", "")
};
byte[] responseBytes;
try {
    responseBytes = client.UploadValues("https://api.stripe.com/v1/charges", "POST", collection);
} catch() {

}
var responseText = Encoding.UTF8.GetString(responseBytes);