Those exercises require you to move the implement around the body, which inevitable results in that implement making contact with the body. The sandbag is soft and allows you to move without worrying about getting smacked by a weight or medicine ball. I have tried those exercises with other tools and they don't work as well. It is like using a dumbbell for swings or Turkish Get Ups - you can do it but something about the kettlebell changes the whole dynamics of the exercise that once you experience you realize how inadequate the dumbbell was. Same thing with those exercises and the sandbag - something about the unstability and nature of the sandbag add something unique to them.

The twisting is through the hip, not the knee. There is very little added pressure to the knee joint in that position. In fact, lack of mobility at the hips, which is essential for that exercise, is one of the main reasons people blow out their ACLs. Your knees want to be stable and you want to move from the hips but if the hips don't move then the knees have to take up some of the slack. It is a great way to teach the movement pattern needed behind moving rotationally at the hips without driving that rotation through the knee.

While I understand where you are coming from my opinion is that ss long as your lower back is not moving then it is fine. In fact, it is a movement you do on a daily basis when picking up a kid beside you or getting a bag of groceries out of the car. It is a great way to teach you the mechanics behind proper rotating and bending and perhaps help riders avoid low back problems in the future. Rotating is part of our natural movement patterns, we just have to make sure we are rotating at the right areas and stabilizing with the right areas. The low back is meant to stabilize, not move, and if you substitute rotation at the hips for rotation at the low back you run into big problems. Using a 35 pound (it can be even less if you need) sandbag to groove and strengthen those rotational patterns is an essential part of a training program.

The rotational lifts don't work with the kettlebell since it would bang into your legs and hurt a lot more than a sandbag.

I was a NCAA D1 track athlete....

Huge fan of the Convict Conditioning book, these exercises were taken directly from it.

Space and time keep me from covering everything I would in a full program. On my site I have several mobility drills, here is one of the more popular ones: http://www.bikejames.com/mobility-training/warm-up-drill-for-a-workout-or-mountain-biking/

Like I mentioned in the video your nervous system sees controlling your bodyweight differently than it does controlling an external load and you need to be adept at both. Being able to do 5+ handstand presses and being able to press a 70 pound kettlebell overhead are two sides of the same coin - you must have both to be truly strong and functional. The bodyweight master who can't deadlift a decent amount of weight is no better than the guy who can squat 200 pounds but can't execute a pistol squat. Everyone wants an "either/ or" answer but it isn't that simple, you need both in your program. And some interesting research by the guys at Functional Movement Systems has found a correlation between a high score on the Functional Movement Screen and reaction time - apparently when you move efficiently you can react faster. And I am a big advocate for tumbling drills for mountain bikers, I have a couple of them posted on my site. BTW, bench press has to be the most worthless indicator of athletic prowess in the world. If you are lying on your back pressing something off of you then you already f'd up. How much can you deadlift, single arm shoulder press, do a chin up with or a Turkish Get Up with? All of those exercises are much better indicators of fitness and overall strength. Bench press has a place but it is a bodybuilding tool to build big pecs or a lift in the sport of powerlifting - if you are not a bodybuilder or powerlifter then it should not be a focus of your program.