Why and how does a drill hammer go half the rpm when put in reverse?
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That actually depends on the motor type.
For DC motors the speed and direction are directly controlled by the voltage you supply. So half voltage reverse polarity will give you exactly that result.
For AC motors and brushless motors the speed is controlled by the switching frequency of your voltage, so can be electronically set, and the direction is controlled by the order in wich the different coils are supplied with current.
Because motor coils are inductors, it takes a bit of time for the field to build up. Thus, for high-speed use in one direction, you can bias the position at which the commutation happens with respect to the magnetic field. If the commutation leads a bit, the motor will generate more torque for the motor current, and it’ll go faster — in that direction.
I believe they are called brush timings. It is controlled by angling the brushes. Since drills usually operate primarily in forward direction it makes sense to optimize torque in that direction
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