driver-core/src/main/com/mongodb/BasicDBObjectBuilder.java
public BasicDBObjectBuilder push(final String key) {
        BasicDBObject o = new BasicDBObject();
        _cur().put(key, o);
        _stack.addLast(o);
        return this;
    }
public BasicDBObjectBuilder push(final String key) {
        BasicDBObject o = new BasicDBObject();
        _cur().put(key, o);
        _stack.addLast(o);
        return this;
    }
Creates an new empty object and inserts it into the current object with the given key. The new child object becomes the active one.

@param key the field name
@return {@code this} so calls can be chained