Stupid question time.

I was watching in play betting on a few races, and couldn't work out how odds changed so quickly when it takes so long for a bet to be placed. Is it possible to place a lower than current odds bet, so it is placed only if the in play odds drop to the chosen odds. For example, a horse starts at 4.5, but I only want to bet if he is running well and his odds drop to say 2.0, can I set up a bet at 2.0 whilst his odds are higher? Would certainly make winning a bit easier!