
a very good question u asked bro!
however by istibra maybe u mean istinja?
the rule is that istinja is necessary only when something is excreted from the privates i.e front and back...whether that maybe liquid e.g urine sperm or any other liquid or solid form e.g excretion.....whenever istinja is necessary then wudu will be nulified.
note. many ppl r not careful when doin istinja which results in drops of urine or mazi coming out later on as a result wudu will be nulified!!
Allah knows best