Session management and sliding expiration

default session time out is 20 minute. if some one interact with web page just before session time out which has been idle for long time then what will happen. session time out will receed i mean increase or session will time out after 20 minute.
 
i hard session time out is based on sliding expiration. so tell me due to sliding expiration session time out increase ?
 
if increase then how much it will increase...is it next 20 minute.
 
please discuss about the nature of session time out and how sliding time out works. how session time out increase due to sliding time out ?