I wonder where and what you have been reading?
The very first para says,
It is often not possible to decide an application for an extension of leave until after the
period of leave has expired. To prevent applicants from becoming overstayers
through no fault of their own, section 118 of the Nationality, Immigration and Asylum
Act 2002 introduced an amended section 3C into the Immigration Act 1971. Section
3C automatically extends the leave of a person who has made an application for
further leave to remain during a period of extant leave. Technically, the leave is
"treated as continuing".
To benefit, a person must have existing leave to enter or remain at the time when
their valid application is made. Section 3C then prevents such an applicant becoming
an overstayer during the period in which their application for a variation of leave
remains undecided and, thereafter, while an appeal against any refusal could be
brought or is pending.
If you still do not understand, then you will need lecture from me, not about immigration, but about common sense and english language.
For one such taster of my lectures, you can check out this link.
http://www.immigrationboards.com/viewtopic.php?p=681406