A kettlebell isn’t much to look at — it’s basically a cannonball with a handle.

And yet, this decidedly low-tech device has become a hot fitness trend. Kettlebell guru Jessica Huthmaker swears that the cast-iron weight is a key that opens a world of fitness opportunities as you swing it, fling it and hoist it.

A 300-year-old workout invented by Russian strongmen and later adopted by the Russian military and Olympians, the kettlebell is getting a new life among American athletes and fitness fans. Thanks go mainly to kettlebell missionary Pavel Tsatsouline, who has been preaching the benefits of the humble kettlebell for several years.

But, really, what’s the attraction to flinging around a bowling ball for a half-hour?

Huthmaker lists the benefits, as she sees them:

It’s fast. It is an efficient workout, combining cardiovascular and strength and flexibility conditioning. So it saves time.

It’s fun. It offers challenge and variety because the moves can be changed endlessly, and if it gets too easy you can always add more weight.

It’s functional. A kettlebell workout is based on functional movements. You might never use an arm curl in real life, but kettlebells should train your body to handle picking up Junior and the briefcase while chatting on the cell phone.

It’s mobile. Take a single kettlebell to the park or on vacation and you can still do your workout.

It’s safe. Using kettlebells is low impact, and if done correctly, a kettlebell workout is easy on the back.

It beefs up performance. Huthmaker trained elite athletes for many years, and she says explosiveness and agility can be enhanced by kettlebells.

It works the core. It gets results because it is a core workout, and it builds up muscles that burn fat. The basic kettlebell swing burns in the thighs and midsection. But don’t worry — there are plenty of exercises to make your arms hurt.

A kettlebell program can be cheap or expensive, depending on how much face-to-face motivation you need. But trainers are also reaching out to do-it-yourselfers with DVDs.