Resonance
Resonance
[Cueball is sitting at a desk, which is vibrating.] clatter clatter [He leans back and turns to face someone sitting at another desk behind him.] Cueball: Excuse me--you're jiggling your leg up and down. It's traveling through the floor and making my desk resonate. Friend: Oh, I didn't even realize! I'll stop. [Cueball passes a sheet of paper to the friend.] Cueball: Actually, can you just shift the frequency up by 15%? I think you can get resonance with Steve's desk instead. Friend: Uh huh... Cueball: Here are the calculations. Let's coordinate and try to spill his drink.

Resonance is the tendency for an object to oscillate when energy is transferred to it at a specific set of frequencies known as harmonics of the natural frequency of the object. A simple example of this is pushing a child on a swing: by pushing the child at the right moment, more and more energy is transferred to the system and the amplitude of the oscillation grows, making the child swing higher.
In this comic, the friend is jiggling his leg up and down at a harmonic of the natural frequency of Cueball 's desk. This causes Cueball's desk to vibrate more and more until objects on it start to bounce around. Rather than have the friend stop, Cueball wants him to slightly increase the frequency of the jiggling in order to spill the drink on Steve's desk (not pictured). In xkcd, no one likes Steve, as demonstrated in comics 1532 and 1672 as well as this one. However, it is hard to have a person control a subconscious movement exactly, let alone increase or decrease by exact figures.
In the title text, his friend confesses the obvious fact that he couldn't control the frequency very well. (Alternatively, Randall may have made this observation himself through experimentation.)
[Cueball is sitting at a desk, which is vibrating.] clatter clatter [He leans back and turns to face someone sitting at another desk behind him.] Cueball: Excuse me--you're jiggling your leg up and down. It's traveling through the floor and making my desk resonate. Friend: Oh, I didn't even realize! I'll stop. [Cueball passes a sheet of paper to the friend.] Cueball: Actually, can you just shift the frequency up by 15%? I think you can get resonance with Steve's desk instead. Friend: Uh huh... Cueball: Here are the calculations. Let's coordinate and try to spill his drink.
