Just started trying my mare in one of these and so far so good x.
I have turned it upside down though so I can use the shorter shank, (it makes no difference to the mouthpiece what way up it goes) and I use the one rein on the shank part x.
I find a huge difference in effect depending on which way round you use mouthpiece: ridged side on bars = pretty effective airbrakes! ridged side away, really helpful in just getting a bit more respect from young horses who get a bit rude or on their f/hand. I use mine loads, also the snaffle version
i used one for a while on mine, correct size and all but, although it worked to start with i found it seemed to pinch his mouth and make him sore so went back to my 3 ring gag. Sold the TT to a pony clubber with a strong pony though and it worked brilliantly for her
either pelham roundings or if she really has been playing up i will ride her with one rein on the bottom but will usually only have to do that once in a blue moon to remind her who is boss - had to recently in a group jumping clinic as i was getting tanked around.
i dont use my tom thumb too often though only when shes getting a bit strong. i have the sweet iron/copper mouthpiece version.