Transitional heat cycles happen in the spring and fall when the sun causes a hormonal change in the mare's brain. This hormone starts a cascade of other hormones which tell the mare to start building a follicle. However, the first few times she doesn't produce enough of the hormones to cause the follicle to ovulate. As the hormones wain, the follicles die and go away. Since the follicle never actually ovulates, no CL is formed and therefore there is no progesterone to tell the mare to go out of heat. Since follicles produce estrogen, which tells a mare she's in season, as the mare comes and goes out of heat she will cycle between actively seeking a stallion and being indifferent (but usually still allowing him to breed if he wants).
The long, long heats typical of spring transition are caused by the small amounts of estrogen and no progesterone. Once she ovulates for the first time of the year, she will generally start cycling normally. Until she ovulates, no pregnancy is possible.
As far as the "floating" idea, yes, that is true. For about 20 days, the embryo will migrate around the uterous. This is MANDATORY for pregnancy. If the embryo doesn't move around the mare will not recignize herself as pregant. Once the embryo is large enough to get "stuck" in the folds of the uterous, it will remain independant for an additional 20ish days. Around 35-40 days it will formally attach. However, it is the period around 20 is when the mare officially recignizes herself as pregnant, and keeps her CL intact. This CL produces progesterone, which keeps her out of heat and acting like she's pregnant. It starts producing its hormone a few days after ovulating the egg. If that embryo can not move around enough, she will kill her CL and start building a new follicle with a new oocyte (egg).