Conservation of momentum

with links to video
It is important to memorise the statement.
The total momentum of a system is a constant provided no external force is acting.
Concept wise :
If you are running with a bowling ball , the system is you and bowling ball, momentum is mass (you+ball ) x velocity. You stop abruptly to allow all the momentum to go to the ball. In that case , the velocity of the ball would be large. Your coach would be happy to see you transferring your momentum to the bowling ball. In other words, initial momentum(before throw) is equal to final momentum(after throw) .Momentum is constant (not lost) in a system because the force on ball and force on you which is equal and opposite and therefore justifying the application of conservation of momentum(no external force is acting) .
If you recall , many throwing events , the athlete has to increase his momentum before he throws and transfers all this momentum to the shotput , discus, javelin, tennis ball , cricket and many other examples.