A 4-stroke engine goes through induction, compression, combustion 
and exhaust strokes to generate power. Before the advent of VTEC, 
the valves controlling the intake and exhaust strokes were operated 
according to fixed rules.

If the intake valves were made to open a relatively small amount to 
privilege drivability at low engine speeds as used in normal driving 
conditions, the engine would not be allowed to intake enough air at 
higher engine speeds, sacrificing outright performance. On the other 
hand, if the intake valves were made to open wide to privilege breathing 
at higher engine speeds, performance at low engine speeds 
would be compromised. This is a dilemma that has plagued engines for 
over a century.