How do you slap on an acoustic guitar?

I know not that many people in this forum are into acoustic guitars, but the other day I was trying to learn this song called 'chatterbox' by Tim Reynolds and the tab said you have to slap the bass E string for some of the parts.
I haven't really played the bass guitar so I don't quite know much about slapping other than that you use your thumb and hit the string real hard.
Can anybody help me?

I use this technique in quite a few acoustic songs. Unfortunately, it's not something that's easily explained. You've really got to see someone do it.

I'll try explaining it anyway, and by the way, this isn't necessarily the "right" way to do it, it's just the way I do it.

Put your index and middle finger together and place your hand above the strings (like you see bass players do) so that your two fingers are perpendicular to the strings. Then quickly thwack the strings with the flat of your two fingers just before the sound hole (or whatever it's called) and lift them so that the strings ring freely.

The trick is doing this in the middle of a song where you have to switch between strumming and slapping. It takes practice.

Thanks Christoph. I just tried it and it's working somewhat and looks like it's gonna take alot of practice to incorporate the skill in the middle of the song.
Is there any way I can slap it using the thumb, because the particular song I'm working on right now demands my index and middle fingers already occupied on other strings and to slap simultaneously. I've seen some bass players use the side of their thumb for slapping so I tried it on my acoustic and all I can hear is the metal string heating the neck instead of the thwangy melody.

I've never really used the thumb for this technique, but if you want to try it out, rest your hand behind the bridge of the guitar for leverage and then slap the bass strings with the side of your thumb.

The problem with doing this on an acoustic guitar is that back by the bridge the strings are real "stiff", or don't have a lot of give, and you won't be able to get much sound out of it. You can try slapping the strings with your hand resting in front of the bridge and further forward, but I don't think you'll be able to get the right sound out of it without the proper leverage.

I think practicing the two finger slap would be a better way to get the sound you're looking for. I've been doing it for while and can do it almost second-nature in the middle of a song. It just takes pratice.