Quote Originally Posted by RedFox View Post
Пойду + 1st person singular of the future tense perfective expresses a decision or an intention to do something, similar to the English phrase "I think I'll...":
пойду покурю;
пойду позавтракаю;
пойду посплю;
пойду сделаю уборку;
and so on.

On the other hand, the particle -ка attached to 1st person singular of the future tense perfective is used to express the same idea:
напишу-ка ему письмо;
куплю-ка я эту книгу;
and so on.

The meanings of both construtions are similar, but there are some differences:
Пойду is more like "I have an intention to do something and I think I'll do it". -ка is more like "Right now I have made a decision to do something".

Also, пойду can be used only in cases when a person actually goes somewhere to do something. For example, if you are planning to buy a book and you are not in the shop yet, you can say both "Пойду куплю ту книгу, про которую рассказывал Вася" or "Куплю-ка ту книгу...".
Another situation: you are keeping a book in your hand and doubting whether to buy it or not. Then, you finally make a decision and say: "Куплю-ка эту книгу!". "Пойду куплю" is impossible here.


Пойду-ка брошусь is just a combination of these constructions. The meaning is basically the same, but with slight emphasis on the fact that a decision taken was sudden.

Пойду-ка брошусь с моста в реку. — I think I'll jump off a bridge into a river (to commit suicide).
Understood ... It means right now