Not sure what you mean by a free spinner engine.
A boxer engine is an engine with two cylinders or a multiple thereof (eg 4 or 6 cylinders) that are horizontally opposed and the corresponding opposite pairs fire simultaneously
As a result the boxer engine is one of the three engine types with a natural dynamic balance The other 2 being the V-12 and the straight-6.
However on boxer engines with less than 6 cylinders there is a noticeable torque reaction, especially noticeable on BMW boxer engine motorcycles, due to the cylinders not being directly opposite but slightly offset from each other