To be congrent two figures need to have corresponding sides with the same measure.
Non-rigid motion or transformations are those that change the structure of a figure
Rigid motion or transformations are those than doesn't change the structure of a figure.
Then, The two figures are congruent because only rigid motion are used to map Figure 1 onto Figure 2.If you compare the figures you can identify the correspondig points:
W and M
U and N
S and K
T and J
V and L