These have the POWER to shut up the sky, in order that RAIN may not fall during the DAYS of their PROPHESYING;
Power is associated with the Holy Spirit (Acts 1:8). When the anointing is upon someone, they have this kind of POWER. Elijah in 1 Kings 17:1-7 shut off the rain. Elijah's POWER is equated with prayer for sick and confession of sins, James 5:13-18. Note the exact time it didn't rain in Elijah's time....what is the duration of these two witnesses PROPHESYING (verse 3).
and they have POWER over the WATERS
This carries forth the spirit part of power. If water is the abode of spirits, these witnesses by shutting off the water in effect are not giving evil spirits (demons) an abode. See Revelation 9:14. This is another reason why the antichrist is irritated by these two witnesses.
to turn them into BLOOD,
In the Old Jewish laws, there is a direct relationship between life and BLOOD. Atonement comes through the blood into ETERNAL LIFE. Thus, the two witnesses not only have the unique ability to inhibit the evil spirit abode but also to change that abode into life. That is exactly what Christ did for us on the cross (John 19:31-37)
and to smite the earth with every plague, as often as they desire.
This chapter in Revelation may be a retrospective look to the seven seals and the seven trumpets. Much of what has been described up to this point during the first half of the tribulation looks like it was/is done under the power of these two witnesses. What is interesting is that these two witnesses are not brought up in Revelation chapter 4 when the seals and trumpets appear. This chapter is like an afterthought which explains how the seals and trumpets are executed.
http://classic.biblegateway.com/bible?version=NASB&passage=Genesis+3:17-19 'Every plague' denotes that these two witnesses are dealing at a spiritual level. Recall that plagues and diseases are part of the sin curse, Genesis 3:17-19.