Sub-question relevance just works like the question or group relevance. You check the conditions under which you want to show a certain subquestion.
Assuming that the subquestion codes in Q1 are Opt1, etc. you would write something like Q1_OPT1=="Y" into the subquestion relevance field to show a subquestion if in the previous question OPT1 was marked.
Help us to help you!
Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.