Kihon dosa literally means basic movement. It is a set of movements (3 movements in both ichi (irimi) and ni (ura)). It teaches you how to move correctly in your techniques. Theese movements are very short (seconds) unlike the karate katas for example. But they are included in every (Yoshinkan) aikido techniques.
First movement is Tai no Henko. It is about changing direction of movement using hips power. Second movement is Hiriki no Yosei. It is called elbow power. Again movement from the hips and the use of elbows. And the last movement Shumatsu Dosa combined theese two previous and it is based on Shiho nage.
Kihon dosa teaches you good posture, good angles of movement, hand position, hips work, rhytm and so on. And because theese movements are presented in techniques, you can improve all your techniques improving your Kihon Dosa.
Another advantage is, after you know Kihon dosa, you will remember the techniques better being a beginner (on seminars for example), because you can translate sensei's movements to kihon dosa (of course except the detail work of hand etc.)
I believe that in Gozo Shioda's books (Dynamic Aikido, Total Aikido) you will find a lot of information about Kihon Dosa
BTW I found another site with good yoshinkan mpegs
Check it out.