Mechanical support includes every type of rifle support from rail gun bases used in benchrest shooting to bone-on-bone contact common in position shooting.
Isometric tension is developed when two or more forces oppose each other to achieve stability. For example: jambing a rifle into a tank trap or against a rail by leaning into it.
Natural Point of Aim (NPOA) is a concept that comes from position shooting, where as you relax into your position, with your eyes closed, the rifle 'naturally' points directly at the target. Because meat and bones are the only support in position shooting, any deviation from NPOA must be accomplished by either making an adjustment to your position, or by muscular input or 'muscling your gun'. Since muscular tension (or 'hold') creates oscillation in the muscle groups used, it is preferable to achieve NPOA without any muscular input. The concept transfers to precision rifle shooting only when mechanical support must be augmented by human physical contact. For example: shooting prone off a bipod without a separate rear support (bag or monopod) requires your shoulder to become the rear support. At this point you should check and adjust your POA to be NPOA by closing your eyes, relaxing into the position, open your eyes. If the target is not under the cross hair, you need an adjustment of some part of your support. This adjustment should not come from increased muscular tension, or trying to hold the rifle on target by muscle power. In precision rifle shooting, mechanical support is encouraged. Use it. In position shooting, mechanical support is not allowed. They are entirely different shooting styles, each with their own techniques. Which is not to say that during a precision rifle match (or a real world mission) you might not use position shooting techniques to your benefit, but realize the advantage provided by the use of support. When time permits, more support equals greater stability equals longer endurance and increased accuracy, both initially and over time.