two questions about acute bursitis of shoulder

hi guys.
1-what is the meaning of this sentence:
"Perform in pain-free ROM with low load, high repetitive format, inhibiting deltoid cocontraction to decrease excessive subacromial compression"
please describe the deltoid co contraction?
2-" usually compression test is possitive in acute bursitis" what is the compression test there?
3-as you know , the tendonitis and bursitis have the very similar signs and symptoms. but in the shoulder what is the differential diagnose between them?