Biphasic pulses consist of a positive and a negative phase, starting with the anodic or cathodic first. The charge balancing phase is usually within the refractory period of the nerve, hence the second phase does not excite action potential, but balances charge only. A time delay between both phases is called interphase gap. As (nerve) tissue acts as a low pass filter the charge applied during the first phase may be compensated by charge from the second phase, without stimulation effect. Therefore a proper interphase gap may improve stimulation efficiency.
A detailed description of the arithmetics background and a step by step guide as a PDF is attached
. Example LabChart files are also attached.