A half of Shami's and Yadav's scalps have been either bowled or leg before the wicket, which clearly depicts their strategy. Bowl within the stumps, on off-stump, and make the ball deviate a shade away or into the batsman.
Stump to stump: The two pacers have banked on the ploy to scatter a few around the fourth-fifth stump, before slipping one around off-stump. The wisdom is manifold. With not much of lateral movement available, pitching outside the off-stump is often a fruitless exercise, as the batsman could leave without much fuss—you don't often see batsmen nicking behind. Pitching on stumps would make them invariably play their shorts, besides bringing the lbw and bowled into the picture, more so if the bowlers can clock 140kmph on a consistent basis, as Shami and Yadav have been. It's the reason most great subcontinental bowlers, like Waqar Younis and Wasim Akram, have truckload of bowled and lbw. Like them, a one-third of Shami's and Yadav's overall wicket-tally have been either bowled or lbws. Moreover, as the strip starts deteriorating, the bowlers could exploit the variable bounce the subcontinental tracks are prone to. So it's a win-win situation unless you stray onto the middle stump.
Marginal Deviation: Green-tops are rare in sub-continent. So the bowlers rely on seam movement, decisive than massive movement. For example, most of Shami's "clean bowled" came from deliveries that held its line or moved away minutely from the right-hander. Batsmen don't expect movement at all, except when it's reversing, so they end up playing the wrong line, as South Africa's batsmen were routinely in the series. Shami achieves this marginal away movement with the help of a beautiful wrist position and a clean release (he doesn't whip the ball like conventional seamers), which ensures that the seam remains upright in the flight. It doesn't wobble and lands on the seam
Likewise, Umesh has worked really hard on his release, he doesn't wrap the wrists as tightly around the seam as he once used to. He always had a good out-swinger, an offshoot of his action, and now he has learned to use it effectively as well, especially against the left-handers from over the stumps. He concedes the impression that the delivery is sliding with the angle, only for it snake back in. It would be the stuff of nightmares for Quinton de Kock, who was repeatedly given a working over. The movement, yet again, is subtle than pronounced, reinforcing the truism that you don’t need massive movement to get batsmen out.
In-between length: As crucial as the off-stump line is the length. Neither quite good length nor back-of-length. Neither drivable on the front-foot or defendable on the back-foot. This dilemma has manifested in the indecisive footwork of South Africa's batsmen. Freeze-frame any of their dismissals, you can spot that they're caught at the crease, groping at the ball, meeting the delivery way in front of their eyes. It's the instinct that kicks in. Back on hard bouncy South African strips, they could leave the stump-bound deliveries on the length. Here, because of the lack of bounce, they are induced to play at those, but without making the required adjustments.