|kick! kick! kick!
Snazzy new jeans - with real high kick action! So yes, depending on fit and amount of stretch, this can be done in normal jeans -- I find it does eventually tear a hole in the crotch - which ideally won't be happening to these, thanks to the glories of a crotch gusset!
|the gusset in question
Turns out such things are fairly standard on high quality men's workpants - when stretching and moving and whatnot are important. Like armpit gussets, they reduce the strain on the surrounding pieces of fabric, and add crossgrain stretch and motion. To draft one, just take a chunk out of the front and back of the crotch (more from the back, so it doesn't show)
The gusset itself should be cut on the fold, thus eliminating the mid-crotch part of the seam. Remove the green parts from the front and back and attach them to each other to become el gussetto. (Don't forget seam allowances - esp flat-felling seam allowances)
And the best part, is you can't see the gusset during normal wear, being all tucked away and invisible (granted, if you start doing cart-wheels and what not, which is kind of the point of the whole thing, then, yes, it will probably show)
[short rant about the impossibility of properly photographing dark fabric without a professional lighting set up in the evenings in winter - you know how it goes]