You can use the unhold command to control which UDFs run when continue commands are entered. Threads that are held will not execute; all others will execute.
unhold all unhold thread-selectors [...]
Where syntax element thread-selectors specifies a string that selects one or more UDF threads being debugged.
You can use hold and unhold together. For example, you could use hold all and then unhold just the thread you want to execute.
This affects only the continue command; the step and next commands implicitly hold all threads except the current one.