I've observed how some Spektrum tx and rx combinations will put the tank to sleep when you turn off the radio, while others do not.  Here's my theory. 

Spektrum has used DSM and DSM2 tech (I purposely omit DSMX, their newest).  DSM was the Spektrum DX6 radio and receivers AR6000 and BR6000.  Spektrum's first generation.  Then came the DSM2 equipment,(second generation) which are backwards compatible with DSM equipment (but not vice-versa)  DSM2 txs included the DX6i, and  DX5E. DSM2 rxs were the AR500 and the AR61xx series. 

After doing some tests, my theory is that tanks running both a DSM2 rx and DSM2 tx will work as a sleep switch by turning off the tx.  However, DSM2 tx and DSM rx mixed systems do not put the tank to sleep.  

Anyways..., it is just a theory.   <br type="_moz"/>