How Does the Crew of an Apollo Mission Open The Transfer Tunnel Between the CM and LM?

The docking between the Apollo command and lunar modules employed the use of a "probe and drogue" system. The probe was mounted in the docking tunnel of the command module, while the conical drogue was mounted in the docking tunnel of the lunar module.

The CM would extend its probe and approach the lunar module. The cone of the drogue would guide the command module until it was properly centered, and the capture latches on the tip of the probe would engage, effecting "capture." The probe would then be retracted until the docking tunnels of the two spacecraft met, and the 12 docking latches on the command module would engage the LM's docking tunnel, effecting a "hard dock."

Once hard dock was achieved, the probe was collapsed, removed from the docking tunnel, and stowed. This allowed access to the LM's drogue, which was similarly removed and stowed, allowing access to the LM's pressure hatch.

It was opened from the command module side. The entire hatch was then removed and stowed leaving a completely open tunnel. I once read that Michael Collins, on the Apollo 11 mission, was pretty concerned that he wouldn't be able to get the tunnel open once Armstrong and Aldrin returned and that they'd have to do a space walk to get back into the CM. He said there was a tool kit onboard that he was supposed to use to basically partially disassemble it if it got stuck. He said that he couldn't even repair the broken door latch on his screen door at home, so he certainly didn't think he'd be able to repair a stuck hatch in lunar orbit.