It reduces the manaloss when you get hit while having ES active.
Say you get hit by a monster doing 100 damage and your energy shield is at 50% absorb, without TK you would then lose 50 life and 100 mana, if you have lvl 20 base TK and 50% energy shield taking the same amount of damage, then you would lose 50 life but only 37,5 mana.
There is a chart for this somewhere but I'm too lazy to look it up, hopefully you get the idea anyway.
Well, at lvl20 TK any hit taken to E-Shield will drain .75 mana per damage. So you will be taking less damage to mana than the 1 to 2 ratio without TK. But one of the problem of it is that it doesn't count in your resist and it just goes with the standard 0 resist.
The chart only provides exact percentages and decimals.
The only things you really need to know is that lvl 1 or 0 telekinesis makes your mana drain twice as much as it should, at lvl 16 it drains the amount it should drain and at lvl 20 your mana only takes 75% of the amount lvl 16 does