Logically, Manchester coding produces a square wave. For H1, IEC61158-2 specifies a current signal of about +/- 9mA into a 50-ohm load. But to reduce the bandwidth of the transmitted signal, IEC61158-2 specifies a trapezoidal waveform with rise and fall times between 0.12 and 0.25 of a bit time.
If the device is bus-powered, it simply modulates its DC current draw with this signal. Non-bus-powered devices may take 10mA, simply to provide a base for this modulation. It is allowed for a device to take less than 10mA, then ramp up to 10mA when it wants to transmit. In practice, at this time, all bus-powered devices take more than 10mA.