What does "Trailing Stop buy Order" on Bitstamp mean?
You might have been confused about what a "Trailing Stop Loss" really is. (Or I do missunderstood your question.)

The answer can be found on Bitstamp's website: https://www.bitstamp.net/article/stop-orders-and-trailing-stop-orders/


  What is Trailing Stop Order and how to use it?
  
  The trailing stop is more flexible than a fixed stop loss, since it
  automatically tracks the bitcoin’s price direction and does not have
  to be manually reset like the fixed stop loss.
  
  For example: Market price of bitcoin is $480 and you placed a Stop
  Sell Order at $450, which is in our case $30 below the current market
  price. Suppose price of bitcoins suddenly increases to $550. You are
  now in chance of “locking in” even bigger profit by manually
  repositioning your Stop Sell Order to higher price.
  
  Repositioning your Stop Order can be done automatically for you by
  enabling “Trailing Stop” function. In our case Trailing Stop Order
  will automatically reposition active Stop Order if price of bitcoin
  increases while maintaining spread of $30 between the bitcoins spot
  price and Stop Order.