I use a variety of drills for FC and always love trying new ones. This from GoSwim is brilliant as it is so effective – even more so with my Grade 5 swimmers age 8-11 than young teens.
The swimmer does 8 kicks then a single arm pull. Then 8 kicks and the other arm.
A Masters Swimmer describes it as catch-up slowed down – but I hate catch-up as a drill because the hands tend to be brought above the head rathe than at shoulder width.