I feel that female masculinity is when a female is unafraid of enjoying something that is viewed as masculine. What this is varies significantly, depending on your perspective, your community's perspective, and your society's perspective. For some, it is found in a woman wearing pants. For others, it may be found in a woman spending a night on the town and "picking up chicks". There is no universal answer to what female masculinity is. It includes what a female does (interests), and how a female does it (behaviour/mannerisms).
I would be hesitant to refer myself as a masculine female. There is some masculinity in me, without doubt, and some femininity as well... but neither of these things really define me - they're just parts of my personality. It is my belief that we all have some degree of both masculinity and femininity, and I would say that this belief was fact if I believed I wouldn't get jumped on for stating an absolute.
Masculinity in trans women can be different from masculinity in cis women, though. We have the potential to be more masculine than cis women can be, for a few different reasons. One is that we usually have a male voice that we have to change - and we don't always change it sufficiently, or at all. Many of us have very typically masculine mannerisms, in how we move, speak, and behave. Perhaps it's the socialization, the testosterone, or the fact that our brains may not be the same as those of cis women's, but it's a reality. It could also be that our often more masculine skeletal structure makes typically masculine movements more comfortable.
In any case, trans women are more susceptible to being perceived as masculine in more ways than a picture can tell. Cis women display these sorts of masculine mannerisms significantly less often. Perhaps related, trans women are also lesbians far more often than cis women are.
Much of the time, the masculinity a trans woman possesses is very male-like.
The difference between trans women who are male-like and female-like is unknown to me, though. I'm not sure why some have a masculine masculinity/femininity, and others have a feminine masculinity/femininity. In any case, I see it as unimportant - individuals have a right to be individuals, and what is done doesn't matter so long as one does not hurt anyone while doing it.
Hopefully, scientific research will one day be able provide some answers and clear up the mystery of the mental differences between trans women, cis women, trans men, and cis men.