Drilling forces
Drilling a hole involves two forces moving in different directions: The drill bit moves in a circular motion to remove material with its sharp, spiral-shaped bit (rotational force), while you provide the second force that moves toward the drilling hole (feed pressure). Impact drills supplement your manual feed pressure with an additional mechanical force on the drill bit, helping you make faster progress in hard materials like concrete. 

The impact mechanism
Like the name implies, impact drills are equipped with an impact mechanism that you can switch on when drilling in very hard materials like concrete or stone. A special mechanism in the gearing pulses the drill chuck (and thereby the drill bit) forward at a rate of up to 48,000 times per minute. These forward movements help you get more work done. But be careful! As universally useful as an impact drill is, the impact mechanism itself may only be used when drilling into masonry. When drilling in metal, wood, plastics or other more sensitive materials, always turn the impact mechanism off. This rule applies also to screwdriving, sanding, or polishing.

Mechanical speed limitation
The ability to limit the maximum rotational speed is useful when drilling in sensitive materials. That is why the main switches of many tools have a small wheel in the switch trigger. This small wheel prevents you from unintentionally pulling the main switch completely back. The further you rotate it anti-clockwise, the shorter the so-called control stroke and the slower the maximum speed becomes. Attention: If your machine has a wheel like this and you are no longer satisfied with the machine's performance, please check and, if necessary, change the setting of the wheel before bringing the tool in for service. If a machine does not have enough power, it is often because the user had limited the maximum speed the last time the machine was used.

The electronics
Modern impact drills electronically control their rotational speed. When using the tool, you will notice that the main switch works like a gas pedal in an automobile. The farther back you push the switch, the faster the drill chuck and rotates. 

Electronic speed limitation with Power Control
The larger Bosch impact drills limit speed not through a small wheel, but electronically with the Power Control function. Accordingly, Bosch models that are equipped with this component, such as the models PSB 1000 RPE, PSB 750-2 RPE and PSB 1200-2 RPE, have a setting wheel on the top of the tool. Set the sliding switch to the drill symbol and regulate the speed with the setting wheel.

Electronic torque limitation with Power Control
If your tool has Power Control, you can decide if you want to set the maximum speed or the maximum power, also called torque. The ability to limit torque is important when driving screws into wood, for example. Torque limitation automatically prevents over tightening the screws. Without this function, you could lose your clamping power. The torque limiter is also helpful when you want to drive many screws and tighten with regular precision. In addition, Power Control acts like a safety clutch: if the drill bit gets stuck, the machine will not try to rotate in your hands. By doing this, Power Control prevents the drill bit from breaking or the material from ripping and the gearing from becoming damaged. Set the sliding switch to the screw symbol and regulate the torque with the setting wheel.

2-speed gearbox
Many Bosch impact drills have a 2-speed gearbox. This works like an automobile transmission by offering the choice of more power but lower speed in first gear or alternatively, high speed with less power in second gear. When operating in the higher speed, the ratio of speed to power (torque) is similar to that of a machine with only one speed. By switching back to the lower speed, you are simultaneously choosing greater torque. You will need this when inserting large screws (or when using core cutters, and for stirring). The lower speed allows you to complete these tasks more efficiently and without damage to the power tool, accessories or the work piece. For these reasons, the high-performance impact drills are usually equipped with a 2-speed gearbox.