Why do parallel diode clippers work
As pointed by others, I hoped the question would be clearer.
Though, I guess that the circuit you are referring to is the one below and you are wondering why VM1 reads 0.7V (P.S. not always 0.7V):






simulate this circuit &ndash; Schematic created using CircuitLab

You can understand this from the characteristics curve of the diode:

The diode cannot work at any point not in the diode characteristic curve (The dark blue curve). So the diode 'forces' the voltage source V1 to operate at D1 forward voltage (0.7V for example). If the voltage source cannot operate at 0.7V, a short circuit will happen and the diode OR the voltage source will get damaged and burn.