Canon Rebel T2i Quick Start Manual page 121

Slating for Synching
If you are recording audio for video with an external
recorder, the audio will have to be synchronized to the
image with editing software.
Some software can synchronize the audio to the guide
track audio on your video file. For example, if you used
the low quality camera microphone on the front of the
T2i, this might suffice as a guide track.
But take no chances, always slate each video clip that
you record. The CDA has a few proper cinema slates,
but here I am improvising by using two pieces of wood.
Many people use a hand clap. The next slide has some
tips for slating.