The cover up against right low kick should be done with the left calf. While executing the defense, the boxer puts upper part of the calf under the kick (in knee area).
In the basic position, the boxer quickly elevates his left leg, turning its ball as much as possible to the left. The calf is at right angle to the floor, the ball expanded. Left leg is tense while doing this defense, and hands cover the head from the punches.
That said, some Thai boxing schools have the opposite opinion about the ball position and presume it should be expanded upwards as much as possible.
The right calf cover-up is done is a similar way.
Maincover-upmistakes:
Left leg is bent in a sharp corner, or is not tense, or is raised too high. This mistake makes the opponent to get through the defense and land a kick on the inner thigh part, or the calf muscle, or the right knee;