Jahr literally denotes being manifest or audible. Technically, it describes restraint of the breath at the consonant, in contrast with the audible escape of breath in hams. It is not simply a command to recite loudly.
The letters other than the ten letters of hams belong to jahr. Examples include ب، د، ج، ع، م, while ف، س، ت، ه belong to hams.
Contrast: compare «أَتْ» and «أَدْ». Their articulatory area is close, yet the breath behavior differs: ت is marked by hams, د by jahr. This is a quality of the consonant rather than a matter of speaking at high volume.