Word 1: Rappel (ra PEL)
Meaning 1: To descend (as from a cliff) by sliding down a rope passed under one thigh, across the body, and over the opposite shoulder or through a special friction device
Example Sentence: Despite losing one of her of her shoes she rappelled down the 30 feet vertical wall.

Word 2: Repel (ri PEL)
Meaning 1: To drive back.
Example Sentence: We had to repel the mosquitoes by lighting a mosquito coil.

Meaning 2: To cause aversion.
Example Sentence: Ugliness of any kind repels her.

Meaning 3: To force (something) to move away or apart.
Example Sentence: Usually two positive electrical charges repel each other.

Example Sentence using both words:
The soldiers rappelled down the mountain to repel the invaders from behind.