So here I'm going to demonstrate how a hemiola works. I'm starting with a very simple 3/4 rhythm consisting of a quarter note followed by two 8th notes finishing with a final quarter note. This gives us an unmistakeable 3/4 sound.
When played in 4/4 this 3/4 rhythm moves over the barline and starts on a different beat each time it's played before completing itself. This is shown in the music image below. I've altered the stem direction on each repeat so you can clearly see the original 3/4 rhythm. Note how it moves back 1 beat each time it repeats.
Pay special attention to the fact that I'm counting out the beats whilst playing the rhythm. This is very easy when counting in 3/4 but much harder when counting in 4/4 and something which should be practiced as much as possible. Check out the closing comments and then move onto the exercises.