Relatively low energy, cardioversion shocks need to be synchronized with another part of the heart's cycle in order to prevent delivering a shock (the "R") on the T wave. This is called synchronized cardioversion, and can be sync'ed with the heart's own R wave by pushing the "sync" button on the defibrillator prior to the shock. This places a small triangle above the R wave on the monitor, and cardioversion shocks delivered at that time will be timed to NOT fall on the T wave. To perform synchronized cardioversion, the 3 lead EKG wires from the defibrillator must be on the patient as well as the defib pads! If for some reason, in the unstable patient, the monitor will not sync, it is advised to deliver an unsyncronized shock at the higher, defibrillation doses.
The heart can also produce its' own R on T, occurring when a PVC is initiated on the T wave, causing a V-fib or V-Tach. The risk of this increases substantially with a disorder called Prolonged QT syndrome, when the refractory period of the heart is longer than normal.