This is a pretty cool effect, one I picked up off a forum somewhere, sometime ago (either Harmony-Central or Ableton Forum).

Load up a something with a lot of rhythmic content as well as tonal. The more the better.

Turn loop on.

Set start to 50%. This will allow you to modulate the loop fully.

The loop parameter should be set between 50-60% for the smoothest results.

The length should be somewhere between 1-5%, and the fade set to 100%.

Press a key and listen. It should sound like a soft buzz, nothing special at this stage. Once your start playing with the 'start' knob, you should hear something resembling a time stretch effect, very gritty! The trick is that, as your playing with the time stretch, this means that you can easily be sync your new beast to your project tempo!

Now try resetting the start knob to 50%.

Make a new midi clip for the simpler instrument. Draw an envelope for the start parameter, making it the same length you want your loop to be (eg. one or two bars). Draw the envelope from -50% to +50% in a simple ramp.

Press play.

Every sound you play should now be in sync with the project. You can use this effect for a lot of varied stuff, but my personal favorite is to use it with drum loops. It can totally mess them up!