This is a classic scratch done by sucking in air against either a hand or a finger. This is one of the most realistic sounding and effective scratches there is for a beatboxer to do.

How it's done

Fat Tony's Method

1. Make a kissy face
2. Put your finger against your lip (or, indeed, any part of your hand / arm / foot)
3. Inhale whilst breathing words like "too" or "tooka"
4. Use uvular oscillation and clickrolls to add rolls to the scratch
5. Put into a beat

Note that there are a massive variety of ways you can do this sound. The finger and thumb method described is the most common when it comes to putting the sound into a beat because it's possible to do it whilst holding a microphone. But really, you don't even need to limit yourself to using your hands.

Method 2

1. Place the tip of your finger (You can use any finger) just against your mouth.
2. It's important to leave the gap in your mouth small (size of a pea) so you can get that whistly sound you want. If you leave the gap bigger you will get a deeper scratch which is still a good sounding scratch but not the one you want.
3. Now what you have to do is breathe inwards and you should get that scratching sound.
4. Ok now this is the most important part of crab scratching - changing the tones. To change the tones it is like saying "Tooka" but you do it inwards and unvocalized and this should get your crab scratch sounding great.

Video tutorial