ECG wave that causes ventricles to contract

S-T segment of ECG shows ventricles contraction.

Explanation:

The ECG is used to measure the electrical signals that controls the contraction and relaxation of heart, most commonly known as heart rhythm. S-T segments are the region after the QRS wave in ECG which is further followed by T waves. The S-T segment is generally shown as a straight line as no electrical signal pass through it and this is the time when ventricles contraction in the heart takes place.