This is yet another very good drill to learn the importance of
momentum.
a) Two players play and the point starts either with underhand
feed or with serve
b) They play to 21 and scoring goes like this: if player
A wins the first point, he gets 1 point. If player A then wins the
second point in a row, he gets 2 points so his total score is 3.
His next successive point is worth 3 points so the score is 6:0.
If player B now wins the point he gets 1 point because that is his
first successive point. If player A wins the next point he gets
1 point because his previous succession of 3 points was broke by
player B.
Benefits:
 The players learn that the more successive points they win,
the more they are worth. In real tennis the scoring is different
but the emotional perception of the player is very similar. If
one leads 5:1 and is caught by his opponent at 5:5, he feels as
if he is losing. That's because successive points that you don't
win make you feel very powerless. (At least that is true for most
players.)
 Players also learn that they can get out of trouble faster.
If one is behind 15:5 and he wins only 4 points in a row (1+2+3+4=10),
he levels the score at 15:15. Again  in real tennis scoring doesn't
go that way, but in player's minds it is very similar. If one
leads 5:1 and the opponent gets to 5:3, most players become more
tense and anxious.
 This is the way to approach playing from behind. Even though
the gap seems too big, like at 5:1, if the trailing player can
win two games in a row with many successive points, then the leading
player will feel as if he is already losing. As you can imagine,
playing from that mindset leads to poor results.
