Not all bigeminy is bigeminy

It may be Wenckebach with 3:2 block.

So, what you see is what appears to be an ectopic supraventricular beat because the P wave is on the T wave. You may also get a wider QRS because the coupling interval is short enough to cause aberrant conduction, and then you get a pause.

It is this pause which is the key.
The pearl is that you must look very hard here to make sure that there is not a P wave in it (easy if the P waves are big, but often they can be very flat).

The other thing to look for is the duration of the pause - if it's:
1. A VE - will block the AV node to the sinus beat but will not reset the sinus cycle - will give a pause that from the R wave of tbeat before the ectopic, to(2) the R wave of the beat after the ectopic, is exactly 2x the sinus cycle. This is called a "fully compensatory cause".
2. An SVE, because it resets the sinus cycle, will give a non-compensatory/LESS THAN COMPENSATORY pause
3. Heart block, will give a compensatory pause because it does not reset the sinus cycle.

So, with a VE, it's easy to tell because you see the VE.
Therefore the usefulness of this rule comes in when you are evaluating whether it is an SVE or a AV block.