Watch the Left Hand Hi Hat Technique online guitar lesson by Dmitry Lisenko from Percussive Acoustic Bass (Foundry)
This is the same muting we used in a standard slap technique, now you're just hitting all string with your left hand fingers. For those of you who are already familiar with slap, you should know that the left hand hits is an essential technique. If we hit the strings with all our fingers, it will sounds quite loud, and sometimes we need it. But more often, we need a bit softer & cleaner sound. That's why we'll play it only with three fingers. Keep your index finger on fret board muting all the strings, and with the other three fingers try to hit hard. Avoid any notes or harmonics, you should be able to get bright muted metallic sound. Start slowly, and practice this movement.