Basically, Protestants often subscribe to a belief called penal substitution. This belief holds that Jesus atoned for our sins by suffering the very same punishment, in every way, that we deserved - going to Hell. Hell is eternal seperation from God, so they believe that when He died on the cross, Jesus actually went to Hell, and so was seperated from God.
Now there are a lot of problems with this. First, Jesus wasn’t in Hell eternally; He rose on the third day. The answer Protestants will give to this is that since Jesus is infinite, even one second in Hell for Him is enough for all of eternity for us. However, this doesn’t work, because if Jesus was seperated from God, then He wasn’t infinite anymore, and so He couldn’t fulfill eternity in Hell for all of us.
This leads into the real problem with the idea: God is not divisible. God is one God in three Divine Persons - the uncreated Creator. If Jesus ever was seperated from God, that means He would stop being God for a moment, but if He was uncreated, He could not stop being God, because what makes God God is that He is uncreated. That is what seperates God from us - He is uncreated, we are not.
The other thing is that that for one thing to be seperated from something, the two things must be different. If Jesus was seperated from God, then He was different from God, so we would have two Gods, not one.
Suffice it to say, if Jesus was ever seperated from God, that would in some way mean that either He wasn’t really God - He wasn’t really uncreated - or that He is actually different from the God He was seperated from, so we would have polytheism.
The way Jesus did save us can be complicated, but it can be explained rather simply.
He saved us in two ways. One is that in the Incarnation, His Divine nature was united to human nature. Now human nature, thanks to the sin of Adam, was a nature that was subject to death. However, when Christ died and that human nature died, His divine nature went along with it, so to speak. He basically ‘filled up’ death by putting into it His infinite Divine nature. Thus, when we are baptized into Him, we are baptized into His death - by being a part of His mystical body, His death has fulfilled the need for our own human nature to die. Also, because His human nature was raised and glorified, by being baptized into His mystical body, ours can be two.
The second way He saved us was by offering satisfaction to the Father. Jesus didn’t merely offer His death on the cross to the Father, but His entire life of perfect obedience - including His death. This is why the book of Hebrews talks about Christ entering into Heaven to appear in the presence of God. You see, in a sacrifice in the OT, two things happened. First, the animal was killed, and then the blood of that slaughter acted as a sort of “key” to allow the priest to enter the tabernacle and offer the animal to God. This is because the death of an animal takes it completely out of human hands and gives it to God. To kill an animal is to send its spirit to God. When Christ died, He also was taken out of human hands and was given to God; this is why the book of Hebrews says that His blood bought Him entrance into the true Holy of Holies, which is Heaven. This is the other reason He had to die - by dying, He was giving His human nature to God - a human nature which had lived a life of perfection. Because He was perfect, this offering plesaed the Father greatly. It pleased Him enough that the Father would give to Christ anything He asked, and what He asks is mercy and forgiveness on us, and the Grace we need to be saved. This is why the book of Hebrews says that Christ is able to save us by making intercession for us with the Father. That act of offering Himself is an act of intercession, because by appearing before the Father He, in His human nature, can ask anything He desires of the Father. This is another reason why the Incarnation is important; it allows a man to ask God for salvation - the man Jesus Christ - whereas if Jesus remained merely Divine with no human nature, He would just be God and would be the one that needed to be asked.
If you want a deeper understanding, please check out the following, or ask questions:
www.soladeiverbum.com/massmain.shtml