Completion can be used to match words used in a document. When typing a word, Ctrlp or Ctrln will match previous or next similar words in the document.
This can even be combined with Ctrl-X mode to complete entire lines. For instance type something like:
This is an example sentence.
then go to the next line and begin typing the same sentence:
Thi
and then hit Ctrlp which will result in:
This
Now still in insert mode, hit Ctrlx Ctrlp and then next word will be completed resulting in:
This is
Continue hitting Ctrlx Ctrlp until the entire line is completed.
If you know you want to complete an entire line type this:
This is an example sentence.
then on the next line type:
Thi
and hit x Ctrll to complete the line.
If the completion being done is a filename Ctrlx Ctrlf can be used to complete that directory. Type:
~/Deskt
then hit Ctrlx Ctrlf and:
~/Desktop
will be completed (if at that location). Ctrlx Ctrlf can then be repeatedly used to list the files in the Desktop.