"Scratching" is the deliberate manual manipulation of the turntables, originated in Hip Hop music. By stopping, reversing and releasing one of the turntables, you can create multiple percussive sounds. Use a short, sharp "swiping" motion to produce a rapid scratching sound. The introduction to the remixed version of "Walk This Way" by Run-DMC features a prominent example of turntable scratching.
Cross-fading lets you mix one track into another without any jarring silences. DJ decks typically have two turntables so that you can play one song and cue up the next. For the tracks to flow seamlessly, slowly fade down the current song and fade in the next song at the same rate. If you simply stop one song and start the next, the flow of the beat is abruptly interrupted. This will disrupt your audience.
The practice of manually altering the playback speed of a vinyl is called beat-matching. It is similar to scratching in that you use your hands to slow down the turntable but the audible effects are more subtle. As one song plays back to the audience, cue up the next song in your headphones. Typically the songs will have a different tempo. By slowing down one of the turntables, you can match the beats of each song so that when you fade the next record in, there isn't a "jump" in tempo.
The equalizer has a series of sliding faders that let you enhance and cut frequencies from the audio. Typically equalizers help you to shape the overall sound of your sound system. However, by temporarily removing the bass frequencies from the audio you can create a dramatic "drop" effect. As the song approaches the middle section, slowly slide the bass fader down to zero. This gradually reduces the prominence of the bass in the audio. Let the track play for an equal number of measures, for example half of a 16-measure verse. As the chorus kicks in, quickly slide the bass slider up to its previous position. The impact of the bass kicking in creates a dramatic and powerful contrast to the previous sound where the bass was minimized.