Biphasic T wave caused by digitalis toxicity
Digitalis (which was by the way originally synthesized from Digitalis Purpurea) has two mechanisms of action:

The action by which digoxin improves contractility appears to be inhibition of the sarcolemmal Na$^+$K$^+$-ATPase &quot;pump&quot;, normally responsible for maintaining transmembrane Na$^+$ and K$^+$ gradients
The major therapeutic electrical effect of digoxin occurs at the AV node, where it slows conduction velocity and increases refractoriness.

So it has a mechanical and an electrical mechanism. About the toxicity:

For example, in atrial and ventricular Purkinje fibers, a high dixogin concentration has three important actions that may lead to dangerous arrhythmias:

less negative resting potential

decreased action potential duration

enhanced automaticity



All quotes are from the book Pathophysiology of Heart Disease by Leonard S. Lilly, 5th edition.
So, I would assume one of the three (or a combination of them) is the reason for the biphasic T-wave. I hope someone more experienced can improve upon this answer.
This maybe has something to do with the relative refractory period ending during the T-wave.
Also, since the figure is from lead I, you can use that information for looking at where the net electrical axis is pointing at during the T-wave. Refer to the hexaxial system.