I believe the Deep Stall happens in two stages. A T-tailed a/c like the 727 can be "conventionally" stalled just like any other aircraft- but if one does not recover from this "conventional" stall quickly the aircraft then enters the Deep Stall from which recovery is impossible. This is what destroyed the BEA Trident after take-off from Heathrow in the 70's- aircraft did not recover from stall and thus entered a deep stall.