"One of the major results if Contrology is gaining the mastery of your mind over the complete control of your body." -J. Pilates
I bet he didn't come up with that quote while transitioning into the Double Straight Leg Stretch in the Series of 5. Regardless of the moment that inspired that quote, you gotta hand it to the man, he is right. And it is at this point in the Series of 5 that you have to tap into that mental toughness.
Quite honestly I have never found it so difficult to lay on my back with my legs straight up in the air as I do when I'm about to do the Double Straight Leg Stretch. Upon first look, everything about the exercise says 'vacation'; you are laying down, your head is lifted but your hands are there to cradle and support it, legs are straight up towards the ceiling, then slowly and gracefully lower and lift the legs.
Meanwhile, back in the fourth circle of Pilates hell, your body is revolting against any sort of movement. Put your hands at the base of the head with elbows wide - when did my arms get so tired?! Keep your head and shoulders lifted off the ground - upper powerhouse muscles seize up in a cramp like you have never experienced. Extend your legs straight up to the ceiling - you have a better chance of getting Johnny Weir to be straight. Lower and lift your legs - suddenly it feels like you have two walruses for legs.
It's mind over matter at this point. As your body physically resists further movement, you have to just concentrate at the task at hand. This won't go on forever, it's fatigue, not pain, (if it is pain, STOP!), no one has ever died from doing the Double Straight Leg Stretch. Focus your energy towards the movement, your body is just having a mild tantrum because it hasn't gotten a nap yet.
All of the sudden you hear 'bring your knees into a tabletop position'. You didn't even hear the instructor count down to the last rep, but it's over, you did it! Your legs gladly fold like a lawn chair and you feel a flash of pride. It's not over, don't go resting your pretty little head, we are at the final frontier, Criss-Cross. Sure, the end is near, but purgatory isn't looking so bad right now...