A clicker is just a marker. Saying the word "Yes!" has the exact same effect if you use it correctly (just like the principles of using a clicker ).

My agility trainer doesn't use clickers since most people don't have three arms, so we use the word "Yes!" instead. It works fine =) Just make sure you use the same intonation, pitch, and do it quickly and efficiently. To measure how well you can do it, drop a tennis ball and say "Yes!" every time it hits the ground. It'll give you an idea of how fast and accurate you are.