Read the FCTM very carefully when it comes to braking (autobrake or manual brake) if landing in a crosswind on a slippery runway. Among other things it cautions on having a directional control problem during the roll-out on a slippery runway. The relevant diagram in the FCTM says it all.
The Boeing 737 advice is to release the brakes and reduce reverse thrust to reverse idle. When directional control is regained and the aircraft is correcting towards the runway centre-line, apply maximum braking and symmetrical reverse thrust to stop the aircraft.
If landing on a slippery runway then most pilots would use an appropriate autobrake setting. Perhaps even max. They would probably use max reverse thrust for effective stopping since the brakes would not be as effective as on a dry surface. If the aircraft starts sliding sideways, then consider how the pilot should release the brakes completely. Pedal pressure increase or switch the autobrake selector to off? Either way, whatever you do, don't cancel the autobrakes by placing the speed brake lever down!
Next move is to reduce to idle reverse. Try that next time in the simulator and note how long it takes to go from max reverse N1 of around 90 percent, to idle reverse of 23 percent. I'll tell you. It takes at least 10 seconds and more. And if you stuff up and quickly place the reverse levers right down to what you fondly hope is reverse idle, the engine will be still delivering around 60 percent N1 forward thrust as the reverser lights extinguish. The last thing you need is all that forward thrust on a slippery runway...
Continue to follow the FCTM advice and try and get the aircraft straight by using rudder and judicious use of brake. Good. Got the aircraft straight by now? That takes a few seconds with no braking going on except for the speed brake drag. By now you have used an alarming amount of runway. Now apply max braking again and full reverse if needed.
Did you know it takes at least 10 seconds to spool up from idle reverse of 23 percent N1 to 90 percent N1 full reverse. In fact, that is why on touch down the N1 stays at 31 percent N1 until four seconds after touch down and if reverse has not been selected by then, the N1 falls back to 23 percent to reduce excess thrust and thus landing run distance.
Sounds too complicated? Well it is - and if landing on a slippery runway in a significant crosswind, you need to know exactly what you are doing in terms of FCTM techique and why. So your questions on manual or autobraking, heels on the floor or up on the brakes, and what if anti-skid is u/s, are all pertinent.
They fade in importance however, when you consider the fancy footwork and reverse thrust fiddling needed to handle a slide sideways caused by weather- cocking and reverse thrust vectors. It sorts the men from the boys.