Climbers with band tension, core on overdrive.
Don't let your hips bounce up, stay flat and let your abs control the pace.
Regular mountain climbers tax the core through speed alone, but a band around your feet means every single knee drive is resisted, and the trailing leg has to hold anchor tension while the other moves. Your trunk's job is to keep the hips dead level while all of that happens at pace. It is a timed drill where the band quietly doubles the cost of every stride.
Only as fast as your hips stay level. The band punishes sloppy strides, so build speed gradually, a controlled quick rhythm beats a flailing sprint.
Around both feet, sitting across the arches so it stays put as you drive. Around the ankles works too but tends to slide more at speed.
Your abs are giving up the fight against the leg drive. Slow the tempo, re-brace, and think about dragging the knee in along a flat line rather than hopping it forward.
