Important theory about rhythm and extensions as a means of shifting between modes
When you go from regular circles to extended circles, the poi has to travel a lot further to complete a circle. You can speed to the poi up to keep making circles at the same pace, or you can keep the poi moving at the same speed and it will take (roughly) twice as long to complete a circle.
2-in-1s is when you keep one poi spinning at one pace, and the other poi at half that pace, so that one poi makes two circles for every one circle of the other poi. You can do this with or without extending one of the poi. Extending the poi tends to make it easier to slow down the rate of circles.
When you do this, you are actually constantly passing between split-time, same-time, and everything in between. This means you can use 2-in-1s to achieve all kinds of transitions!