What do you recommend doing against guys who are very good at doing hug around chest passes or "head spike passes"?
I'm talking about when they pass your half guard by pushing your bottom knee down, spike you with their head or shoulder and use their other arm to control your hips somehow (perhaps underhooking very low near the hip or overhooking the thigh if you have a knee in the hip type of half guard).
If you play half guard a lot, what would you recommend working on to deal with these type of guys?
Get on your side and sweep or close full guard before they get the chance. Once they have this position with your flattened out it's going to be tough to sweep them.
the head is key in that passing strategy - for your opponent's offense and it should also be the focus of your defense.
you've gotta displace the head. push the ear/head to the side towards which your opponent is passing. combine that effort with also trying to frame under with the your forearm on their neck. remember, that they can only resist in one direction at a time. you should also re-enforce those frames by using your hip to bridge and shrimp, stay on your side (keeping your far shoulder up), or even trying to post on your elbow.
at any point you can re-pummel either underhook (depending on what point you are countering this pass) there should be nice reversals there for you as well. make sense
Christian's idea is rock solid. As soon as they wrap thier arms around your legs and hug drive their head in the direction they are passing to. If they get too far and are already past your legs and nearing cross sides, push the head down towards your feet and kill their near arm.(Usually grab it at the elbow and push it under their body) This will effectively shut down their attempt to capture your upper body. They can't finish the pass in this position even though they are past your legs and essentially in a very low cross sides.
If they spike up high where you can't frame and push I'll usually look for a butterfly hook on the opposite side of the way they are passing. When they spike you up high with their head they make their lower body light and the butterfly hook is usually wide open. I'd try to get an underhook on that same side also. Butterfly works great when they've flattend you out and lift the inside knee off the ground. For me, it's a fundamental that when I'm in half guard bottom anytime I'm flat and the top guy lifts the inside knee off the ground I find the butterfly hook. It's almost always there.