see , there are two types of Lock
shared lock --- > read lock
exclusive lock --- > write lock
if your data item is already locked under shared mode then someone else can request for share lock and they can get it
now , if we locked the data item in shared mode then someone else cannot lock it under exclusive Mode because there will be Read - Write confict
but If we locked the data item in exclusive mode then someone else cannot lock it under exclusive or shared mode .
So until the " lock point" No other transaction can modified the data item and therefore , data items not get effected till lock point , after lock point T1 realses all it's lock and unlock all it's Keys then If T2 wants to write and modified the data it will get the key , before the locking point , Transaction T2 not get the key to modied of do any kind of changes in any data item but it's only capable to read the data item