Ultrasonic sensor confusion
What happens is:


You send a pulse of 10 ms with TTL signal. This is the first line in the picture below.
Than you call pulseIn. This waits for the echo pulse.
The echo pulse will become high when reflected. This happens internally in the module. This is the second line.
The echo pulse will stay high depending on the time it took to be echoed back and makes a pin high (echo pin). This is the third line.
Than your pulseIn command returns with the time (of the echo pin length).