[AndyPharmc]

Khalwat Dar anjuman is a mental phenomena,not physical,as Maripat rightly pointed out.Someone who wanted to migrate to Makkah from India asked Haji Imdad ullah ra for advise,the response was:"If your body is in India and your Qalb in Makkah,it is better than when your body is in Makkah,but your Qalb is in India".Obviously remembering Allah swt is a faculty of the Qalb,so for 'khalwat Dar Anjuman' the 'Yad' or consideration of Allah swt should be so strong as to overwhelm all other faculties,which needs practice....and more importantly....complete control over one,s thoughts.
Having said that,the technique in itself is simple.Some one asked 'Bhulle Shah ra' about the reality of tassawuf,his reply was in seraiki language,but it's translation is like this:"How shall I explain,but let me try,it is directing your thoughts from one place to another"- meaning from dunya to Akhera,from matter to abstract,from creatures to creator.
As Dr 76 pointed out,and I take some liberty to rephrase in my words,The Qalb is created from a special material,analogous to Arsh,it has the capacity to be polished with zikr,like a mirror,to reflect tajalli of the creator.Once it is polished,then it is made to take the habit of mushahida by constant contemplation.....and then a stage comes,that even if one wants to focus it on something other than Allah swt,it will not be possible.Gradually this state consolidates,leading to a type of FANA,where one can comprehend nothing but Allah.
A good example of such a state is awareness of our own 'self'.Every one of us is constantly aware of his self,without paying any deliberate attention to it.We do all the daily works,go to office,drive a car,eat,drink,gossip,but although we are clearly aware of 'self',we don,t pay any deliberate attention to our every single act performed by self.In other words,we have developed a constant 'yad-dasht' of our self.Something similar happens when we are immersed in YAD of Allah swt,we remember him all the time,but this does not stop us from any of our activities.If we are lucky enough to achieve this state,this is true 'Khalwat Dar anjuman'.
Zikre Qalbi alone,despite being a big blessing,does not mean yad dasht.As one brother pointed out,once zikre qalbi starts with practice,even if Nauoozobillah one loose his eeman,zikre qalbi may still continue ,what to say of a sin....?.Having said that,one must admit that Zikre qalbi is amongst one of the most potent tools to achieve the objective of real 'Yad Dasht'.