There are two ways to execute test cases in Test Collab:
#1 Sequential method (default): All test cases run one-by-one in a strict manner.
#2 Non-sequential method: While executing the test cases, tester can jump to another cases or come back to the current test case.
You can customize this behavior from Account Dashboard > Settings > General.
To activate non-sequential method (mode #2), check "Disable sequential execution / popup-window for execution"
Note: This is a system-wise preference.