While the DR1's tail wag could partially be the Dutch Roll effect so well described by Mr Hanson (but he didn't say it was Dutch Roll)
Funny thing on rudders - If the total vertical area is "undersized"- such as the DR1.
the tail can wag, as the model simply yaws till the load is peaked - then swing back -then repeats--the rudder can be locked solidly - makes no difference.
The Dutch Roll is typically accompaied by a noticeable roll in addition to the yaw. (which is why its called Dutch Roll)
In the case of THIS triplane... there was no visible roll accompaning the yaw oscillations... and the rudder itself was observed to be flexing. Thus.. I can't assign all of it to the phenomenom he appropriately mentioned.
Something that may be interresting: You can get Dutch Roll at some combination of power, airspeed and angle of attack with just about any prop driven aircraft. Most designs... its found at low airspeed, high power and relatively high (but not at stall) AOA. (Such as my Tiger 60's full throttle appx 45 deg takeoff angle from short field to clear a 50 ft obstical ASAP. This is the only time it shows with this model.)
***************
The information on the "Boost Tabs" is Great. Best explaination of them that I have seen.