waaaaaaaaaa if you mean your opp. then you canot do nothing since your opp has less than 50 also if he has more but no bench still does the damage. does that clear things up.
Scenario 1 : HP > 50 no bench
You may use the move
Scenario 2 : HP < 50 with bench
You may not bench it as the attack says If you do then bench it
Scenario 3 : HP < 40 with no bench
Nothing happens