How do the Tiger Moth's anti-spin strakes work?
The vertical tail of the Tiger Moth is located a bit aft of the horizontal tail due to its sweep, such that the vertical will be in the wake of the root of the horizontal in a typical spin attitude (around 45° angle of attack). I would strongly recommend to fit the aluminium anti-spin strakes - those pilots who had no problem without them flew the Moth at a forward location of the center of gravity, which helps to end the spin. A lighter forward passenger/pilot will make the aircraft behave a lot differently; in spins small differences can translate in a markedly changed spin behavior.



Cut-away drawing of the Tiger Moth's tail. The anti-spin strakes are the parts marked 16.

A spin can be ended in two ways: Either the rotation rate is stopped by neutral rudder or a rudder deflection against the spin, or the separated flow on the wing is reduced by pitching down. The first way works better for aircraft with long fuselages and low wing span, and the second is preferred in aircraft with big wings like gliders. In the end, a combination of both works best and some aircraft need their own technique to end a spin safely.

Generally, it helps to have as much of the vertical tail exposed to undisturbed airflow as possible. Ventral fins are a tremendous help in upright spins but useless in inverted spins. Strakes or fillets ahead of the vertical tail are very effective, especially in fast spins when the angle of sideslip at the tail is high. In the 1930s, Arado in Germany built a range of airplanes where the vertical tail was positioned ahead of the horizontal tail, for the express purpose of making spins easier to end.



Arado 79 in flight (picture source)

The anti-spin strakes produce a vortex at high angle of attack, similar to that on a delta wing. This increases the dynamic pressure on the vertical tail and makes it more effective. Since every bit helps and the spinning characteristics can vary strongly with c.g. position, it would be careless to leave them off.