A current clamp („Stromwandler“ in German) is a simple transformer with ONE primary winding and (in the case of the Shelly clamps) 3000 windings on the secondary side. Thus, a 3000-fold smaller current flows in the circuit at the secondary side. This side must be shunted by a small resistor. Otherwise, the clamp will be immediately destroyed by overvoltage at the secondary side. In the case of the Shelly clamps, a TVS diode is incorporated in the clamp, which limits the secondary voltage to some 10 volts.
Changing the direction of the clamp gives negative power value due to the fact, the secondary current is now phase shifted by 180°. Power in alternate current systems is defined as P = U * I * phase angle (cos φ).
The meaning of „K“ and „L“ is a mystery. In fact, the arrow must show to the source with Shelly 3EM; with Shelly pro 3EM and EM50, the arrow has to be in line with the power flow (it must show to the load).