I’m assuming you know how to do the trick. For me, it doesn’t lose spin at all. The reason it loses spin is because of these:
Bad string tension - Yes, if you have tight string tension, the strings will try their best to wind up around the yoyo, and cause it to snag. It also makes it harder to slide across your fingers.
Sweaty/Sticky Fingers - Yes, sticky/sweaty fingers makes the strings unable to slide across your fingers. In fact, sliding the yoyo in a trapeze is a way to wind up your yoyo. If you have smooth fingers, the string will slide across, and it wont wind up. If you pinch the string or have sticky fingers, it will wind up. Wash your hands before yoyoing, and dry them thoroughly.
Wrong throw - Yes, some people can’t do breakaways, so they do sleepers instead, and this makes the spin of the yoyo against the direction that the string is moving, so it winds up more easily.
Uber Tight gap - Yes, if you have an uber tight gap, loosen it a bit.
When you have these 4 things, the yoyo will do this while skinning it.
http://www.youtube.com/watch?v=ewmf_dG2ZPQ&eurl=http://yoyoexpert.com/forums/index.php/topic,3457.0.html&feature=player_embedded