When I decided to break this yoji down into its baser parts, things got a little confusing. It started with Rikai-chan penning 緩急 as "in case of emergency" when my electronic dictionary told me "to vary speeds". 自在 wasn't much better, with Rikai-chan offering me "freely" where my dictionary's first entry was "universal". I did end up scrolling down to find out that 自在に does, indeed, register as "freely" or "easily", but it's still a curious multiple usage for the word, and it would pay to know what's going to happen if you tack a "の" on the end instead of a "に".
ANYWAY... if you haven't derived the meaning yet, let's have a look-see.
1. The ability to manipulate the condition of something with ease.
2. Freely changing rhythm or speed
The typical application of the phrase is for sports, massage equipment, or pornography. Sports where speed is a big factor, like boxing or baseball, mean the yoji is extremely useful in those situations. Likewise for the latter two examples, though I won't elaborate as I'm sure your imaginations are more than up to the task.
So how to use it out of those three realms? It's not as hard as you might think. What follows are a few clips I thought one could apply the phrase to with varying success...and if nothing else, I guarantee you'll find a few you like.
I want to stress that the great thing about a lot of these yojis is they have adaptability you may not initially think of. A lot of expressions in English are tailored for fairly specific circumstances, but never be afraid to try stretching a yoji one way or another. You may get corrected, you may get oohs and aahs, but either way you'll learn something new.
To me, adaptability is an indespensable part of life. If you want to keep up with all the highs and lows, you have to be able to change your rhythm, so to speak.