To get better at double unders, spend 2 minutes of your warm up practicing them

Death by Burpee
1 Burpee on the first minute
2. Burpees on the second minute
3 Burpees on the third minute
Continue until you cannot complete the corresponding number of burpees in that minute.

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Take care of your hands. Shave your calluses and make grips before a WOD with heavy pull up volume.