Panic button mashing
To be fair, I witness this symptom in other games in addition to World of Warcraft. Do you remember playing Street Fighter or other fighting games when you were younger? You didn't know all the moves. You had a hard time remembering which button was high kick, low punch and block (even if the labels were right next to the arcade buttons). What you remember doing was smashing all the different arcade buttons in the futile hope that you hit some combination that resulted in using a super-special move, blowing up your opponent.
This level of panic isn't going to help, especially as a healer. If you're the person your party is counting on to sustain the raid, then you need to be the most level-headed player there. Actually, both you and the tank will need to have really cool heads. Blindly pressing your keys and fervently hoping that the group doesn't die isn't the best way to go about it at all.
In a client/server type of setup for MMOs like WoW, the game takes the initial command that a player inputs and transmits it to the server. Any subsequent spell instructions are typically ignored or queued with a limited buffer (but it depends on the game). Sometimes, moving or sending a command to the server can instruct the server to cancel the last command that was sent, such as a stopcasting macro. In other words, you might end up casting a slow healing spell only to discover that the situation warranted a much faster spell instead. Guess what? You mashed the wrong key! Unless you can rectify it by stopping the cast and using the right spell, your healing target won't last much longer.
More experienced healers out there know that there is a right heal for every problem. The trick is determining which one, and that's going to be based off the amount of time you have, your mana, the target's health, the encounter and so forth. There are simply too many variables to take into consideration.
Disciplined button mashing
Now we're going to contrast this with disciplined button mashing. To illustrate an example, it ends up being the equivalent of my mashing one spell repeatedly. Targeting a player and then rapidly pressing my group AOE heal spell helps ensure that there is a minimal spell gap between my first and subsequent spells.
Latency (commonly known as lag) refers to the delay between your computer and the server. By repeatedly mashing one key, your computer is going to try to instruct the server to process a specific action. Depending on where you live and your connection speed, you might have noticed that when you're casting a spell, there are times when it looks like your next spell in the sequence is automatically casting, even though you didn't press anything. Maybe you decided to rest your hand for a bit and then noticed that a spell went off.
Due to the delay between your computer and the different server realms, it is highly possible that the server thinks your spell already finished casting. But your side (or the client side) is still playing catch-up because the server is sending information from it to your computer. Even though the spell's done from the server end, your computer is still finishing the cast. If you time it right, you can keep mashing the same key so that your client is continuing to transmit commands over the internet to the server so that it cuts down on the lag time and can process your healing commands faster.
If I'm tank healing, for instance, I'd keep rapidly pressing any big healing spells one after the other. I know I don't really need to. I can probably wait until there's a half-second (or less) on the cast before before attempting to cast another one. Truth be told, it may not even be necessary to do that. You could cast a healing spell and then another one when the first healing spell is less than a second from connecting with your target and still minimize any lag.
Naturally, this kind of stuff takes practice. Smashing the keyboard is a great way to release frustrations, but I wouldn't exactly recommend it as a viable healing method. Start with improving your healing decision making first. As you practice and heal more, your ability to recognize situations will get better and better. Your judgments and decision making skills will become even more sound.
Need advice on working with the healers in your guild? Raid Rx has you covered. Send your questions about raid healing to firstname.lastname@example.org. For less healer-centric raiding advice, visit Ready Check for advanced tactics and advice for the endgame raider.