L sits are a blast! Seriously, one of the most underrated movements we do. People extoll the benfits of the plank, which is great, but for the time spent an L sit is just going to give more bang for the buck.
With a 53/35lb KB complete one KB swing set it down, take one breath, pick up the kb and do 2 swings, set it down, take 2 breaths, then 3 swings set it down 3 breaths and so on all the way to 15 and back down to 1. This is designed to control your breathing and heart rate while under fatigue