Implicit Imsi Detach is not equal to VLR purge.
1)Implicit IMSI Detach = subscriber is marked as detached in VLR; subscriber’s data isn’t deleted from VLR; there is no contact between VLR and HLR; TMSI isn’t froze because subscriber’s data are staying in VLR.
2)VLR purge = subscriber’s data is deleted from VLR and VLR sends VLR purge message in HLR. TMSI may be frozen according procedure. There is no interaction attempt between HLR and VLR when MT call arises and subscriber is marked as purged in HLR.
Pix, when subscriber is marked as “implicit detached” in VLR, there is possibility to establish MO call without VLR-HLR interaction (I think it is VLR implementation dependent to do this with or without Location Updating).