The Stop Entry tool can be used several ways. It can be used to back a price lower than its current price, should it get there. An example may be you want to back Team A at 1.50 should they reach that price. They may currently be 3.00 and you only want to back them when they look certain to win the match. You can also lay them at a higher price.
A horse may be even money favourite and you want to lay it in running when it appears that the horse can no longer win. You can set BetTrader to lay the horse when the LTP (last traded price) gets above say 15.00 by placing a lay at 20.00