Off the top of my head, any sort of "escape" beat is a ventricular contraction originating in the ventricles in the absence of a beat signal coming from the atrium, i.e. SA node. A normal signal to beat, which sets off a well coordinated sequence of events, is called Normal Sinus Rhythm (NSR)
The automaticity (automatic contraction) of the cells in the ventricles is a bit slower than what comes from the Atrium (NSR), That means the atrial signals pace the heart because they are a bit faster. If the ventricular muscle doesn't contract for a longer period than what the SA node usually signals it will contract on its own, and this is called an escape beat because without it, you would experience syncope (pass out). It's a backup in case the SA signal is either blocked or didn't occur.
Escape beats are mostly common to the AV junction or right ventricular outflow tract because these cells have the shortest automaticity (tendency to contract) than all other areas. That just means statistically speaking this is the most likely source of a contraction over time.
If for some reason all atrial signals are blocked, or too slow and the escape beats account for all or large groups of ventricular contractions, it's then termed rhythm i.e. escape rhythm. Junctional escape rhythm means that the ventricular contractions are all coming from the AV junction.
To my knowledge, I could be wrong, the very fact that the word "escape" is in there means that there was either no or very slow signal coming from the SA node (artia) so the ventricles took up the work to pace the most muscular part of the heart, the ventricles.
Since the ventricular sources pace slower, escape rhythm is usually 40-60 bpm. That doesn't mean you can diagnose escape rhythm by BPM!! A lot of healthy hearts will beat down to mid 40s, but it's still NSR.
Very complex to diagnose this on an EKG.