TT Rules Summary from Slowtwitch

Summary of UCI rules relating to TT:

Slowtwitch UCI Rules Summary

Note section 1.3.019:

In that case, the Panel may conduct the following test. Using a
plumb-line, they shall check to see whether, when pedaling, the
point of the rider’s knee when at its foremost position passes
beyond a vertical line passing through the pedal spindle (see
figure 1).


Notes and interpretation:

Slowtwitch TT Dual Use Comments
Slowtwitch TT Fit Comments

Notes from above on passing tests. First for short riders, how to get the seat moved forward legally:

How is such an application made, and how often is it granted? According to the rules, the riders are given a "test" by the commissaires prior to the race. You pedal the bike, and the forward protrusion of your knee cannot pass in front of the pedal axle. Of course, you simply ride on the rearward part of the saddle during the test. As one highly-placed federation technical director put, "Everyone who takes the test passes, and these people get to ride a position unavailable to the naive or unaware."

And for taller riders, how to get another 5cm for the aerobars (80cm instead of 75cm maximum from bottom back extension to end of aerobars):

Taller riders face rule 1.3.023, which says that the maximum distance from the bottom bracket to the forward protrusion of the aerobars cannot exceed 75cm. But, you can get an extra 5cm if you take "the test." This consists of adopting the aero position aboard your bike, with your elbow angle not exceeding 120 degrees. For this test you ride on the very tip of the saddle, and you extend your arms forward to that 120 degree angle.

Here is latest from TDF, outlawing the Landis position:

VeloNews on 2007 TDF TT changes

General bikefit (more Tri than TT):

Slowtwitch Tri Bikefit