In the name of Rabbi Lamm, taken from: http://www.myjewishlearning.com/life/Life_Events/Weddings/Liturgy_Ritual_and_Custom/Ketubah/Details_II.shtml
The Act of Acquisition (Kinyan). In
order to seal all of the stipulated
obligations, and to assure that the
document is not asmakhta (based on
speculation), the rabbis required the
legal formality of kinyan, the act of
acquisition. Because the bride cannot
take possession of all the property,
the groom affirms it by a symbolic act
called kinyan suddar.
Thus, at the wedding, the rabbi or one
of the witnesses gives a handkerchief
or other article (but not a coin) on
behalf of the recipient (the bride) to
the groom. The groom then returns it.
Then they record in the ketubah,
ve'kanina ("and we have completed the
act of acquisition"). This symbolic
act must be seen clearly by the
witnesses, who are the makers of the
contract, before they sign to its
validity. If the ketubah is
calligraphed by a scribe, or printed
in advance of the wedding, one letter
of the word ve'kanina (or the whole
word) is usually omitted so that the
ketubah is technically not completed
before the kinyan itself is made. If
this custom is overlooked, it does not
alter the ketubah's validity, so long
as the witnesses in fact witness the
kinyan-transfer of the handkerchief.