A product concept really only starts to work when the designer has a firm grasp of the hand’s relationship to scale, grip, and contact points. Hold a mug by the handle, then hold it by the body. The object has not changed, only the nature of the hold. One grip is familiar, the other warmer, heavier, and less controlled. This is why industrial design is more than a pretty perimeter around an object.
Grip is not just about comfort. Grip suggests intent. A large handle implies carry, a small cutout for lifting, a roughened area for holding, a flat face for pushing. If these signals are subtle, the object can still work, but it feels more obscure. Vague handling is easily identified by a nice looking form with no obvious point of entry.
Scale alters the meaning almost as much. A speaker, a timer, and a desk tool can all be one-handed, but they have no shared requirements. A speaker should be big enough to seem credible; a timer, big enough for instant read; a desk tool, big enough not to fall over under pressure. When scale is off, handles are too narrow, buttons too small, and the form too light, or heavy.
Contact points refer to the small zones where user and object touch and interact: handles, buttons, lids, cuts, triggers, feet, bases, resting surfaces. Treat these elements before adding detail because they embody the product’s logic. A crease for opening a lid, softened edges for grasping, a raised button for the thumb, a flat surface for resting, and so forth.
Sketch a common household product with contact in mind. Try a remote control, a toothbrush, a stapler, a flashlight, a bottle, or a mouse. Rough out the basic shape and indicate every place the hand can touch, press, push, pull, or hold the object, as well as every point the object rests on another object: on the ground, wall, cabinet, shelf, bag, charger, and hook. It will be a sparse image, but it will tell you the percentage of the product defined by contact.
Redesign the concept by moving one element. Move the button closer to the thumb, increase the grip size, smooth out the lower edge, expand the base size, or rotate the angle of the handle. Keep the design simple enough to isolate the change, so you can see the result. You will likely find a shape more stable and obvious, and easier to handle, but you may also find a problem, such as the crease is blocked or the shape becomes too heavy.
A good product sketch gives the hand a role to play. Before adding texture, tone, or rendering, look at your concept and consider the first three seconds of use. Where does the hand enter the form? Where is the weight? Where does the motion come from? Where should contact be limited or avoided? When you can answer these questions, the idea stops looking like a drawing and starts to look like design.