It's instructive to do that on a trainer or, as I do, on rollers. 2 minutes each leg, enough resistance so that you're crying for your mommy the last few seconds. No slack chain allowed. Made quite a difference to my climbing. Totally not hard aerobically. Say, 10 reps each leg, right leg then left, etc., say a set at 50 rpm and one at 85. It is possible to make it hard enough to be worth doing. I used to do that on group rides which were too slow for me after dropping back to last rider. I'm hoping to get back to that by summer.