I have seen many different spellings for the Hubud drill including Hubbud, habud and Habbud. If you have been practicing some of the techniques in the other posts regarding this drill then by now you are beginning to get a feel of the reason for the tying action name of this drill.
After performing a brush Hair and then the extended arm underneath technique then you can use the extended arm to grab them and then break their arm. As you turn your extended arm over and slide it up the action causes the opponents arm to be driven into your hand. As soon as your palm makes contact with their arm you hook your fingers over their arm and then use your other hand or your forearm or elbow to execute an elbow break on them. This is of course assuming that they are punching on the same side of your head as the Brush Hair that you are using.
You can also use the hand that you can break with to suppress their arm against their body and trap it. You should step forward and keep pressure so that they cannot easily escape. At this point your other hand is free and you can hit them wherever you want with it.
There are other techniques that easily come out of the drill but I have essentially explained the primary moves that comprise the drill.Of course to do the drill you will not break your partners arm or suppress their arm across their body so that at the point where you throw your punch you let them go and they then do the Hubud drill back and you and your partner can continue to do the drill. I recommend practicing the techniques as much or more than the drill so that you do not become to complacent thereby letting someone who just tried to really take your head off have another shot at you unnecessarily because you let them go.
At the same time practice Habud enough so that you get fast at the drill. It will help to build your coordination for other aspects of the art and it will also help you to develop your positioning skills.