The more brainless the activity, the easier it is to do parallel flashcards. An elliptical or stationary bike session will be easier than an equal intensity run where you have to worry about your gait. Flashcards are little atomic chunks for you to process independent from one another. When you do flashcards the only thing you need to concern yourself with is the short prompt on that particular card in front of you. So even if you are working out pretty hard, you can still work your way through a deck, it's just going to be at a slower rate. This differs from reading a textbook or journal article, where you have to keep a certain amount of the previous passages in memory to understand the current line. You will hit a point where flashcards stop working but the ceiling is higher than with other types of learning activities. This will differ per exercise type, per intensity, and the subject you're learning.