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You just start a stop watch. You have until it hits 1:00 to do 1 pullup, then until 2:00 to do 2 more, then until 3:00 3 more etc. So in the 10th minute you have to do 10 pullups in that minute.

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this sounds like a pushup excercise I used to do. You have X seconds to do X pushups each minute for 10 minutes. Beginners use a low X obv. So say your number is 20. Starting at time 0:00, you have to do 20 pushups before :20 then you rest until 1:00. Then you have to do another 20 pushups between 1:00 - 1:20 and rest until 2:00. etc.