What is the ALT INTV on the 737's autopilot?
From the FCOMv2 4.10.10 (emphasis mine):


  Altitude Intervention (ALT INTV) Switch
  
  [Option - With speed and altitude intervention]
  
  Allows manual deletion of next FMC altitude constraint via altitude SEL and ALT
  INTV switch.
  
  Push – (during VNAV climb)
  
  
  lowest FMC altitude constraint below selected MCP altitude is deleted
  if airplane is currently at an FMC altitude constraint, deletion allows airplane to resume climb. MCP altitude must be set above current altitude
  for each press of switch, one deletion occurs
  if MCP altitude is set above current FMC altitude, FMC cruise altitude
  resets to MCP altitude. FMC cruise altitude cannot be decreased using
  ALT INTV switch.
  
  
  Push – (during VNAV cruise)
  
  
  if MCP altitude is set above current FMC cruise altitude, FMC resets
  cruise altitude to MCP altitude and initiates a cruise climb
  if MCP altitude is set below current FMC cruise altitude, an early descent
  is initiated. Lower FMC cruise altitude cannot be entered using ALT
  INTV switch.
  
  
  Push – (during VNAV descent)
  
  
  the highest FMC altitude constraint above MCP altitude is deleted
  if airplane is currently at an FMC altitude constraint, deletion allows
  airplane to continue descent. MCP altitude must be set below current
  altitude
  
  
  [Option - FMC U10.6 and above]
  
  
  if all FMC altitude constraints are deleted during VNAV path descent, an
  automatic transition to a VNAV speed descent is made.