in Baba Harnam Singh ji Rampur Khera's biography, it's explained that the Guru listens with extra close attention when singh's do ardass with hazooria in between their hands.
Wearing hazooria is a sign of humblesness and while doing ardaas, person shud be extremely humble and shud act like a beggar and a sinner.
Covering mouth while doing paath and taking Hukammnama,well thats done so that the saliva of kalyugijeev jeevs does not fall on the Pavan Pavittar Saroop of Pyare Satguru jee.