In my experience, the most common cause of nicks in strops is the timing of the flip and direction change. The timing is critical. Switch directions before flipping and you'll dig the edge in and then flip it up. This will certainly damage the edge. Switching after the flip is complete will dig the edge into the strop. This damages the strop, but may not damage the razor.

While moving the razor forward start the flip, and change direction after the edge is off the leather, but before it comes down again. Practice in slow motion to get it smooth. Remember: slow is smooth, smooth is fast.