thankyou for your patience and explanation
so how does a h-bridge sense that a motor's turning is blocked?
motor is turning a cam
Cam gets blocked by something
....what happens in the circuit to sense the blocking of the cam
When a motor is running, with a load - or if it is stalled (as it is technically at startup) - consider that "infinite load", the amount of current required by the motor goes up; the maximum current is called the "stall current" and is one of the specs of the motor. There is another spec, called the "running current" - this is the current the motor takes when it is running, with no load. With a load (anything that causes the motor to turn slower with work), the current will vary between the "running current" and the "stall current".
At startup, a motor effectively is "stalled", and so the stall current is hit for a very brief amount of time - you need to be able to sense and reject this transient (to allow the motor to run at all). However, when it is blocked from turning for a longer period of time (such as in your proverbial cam situation), then this stall current persists for that longer time.
Typically the way you sense this current is by measuring it with a very small resistor from the outputs of the h-bridge (most h-bridge ICs have these outputs available - generally called current sense outputs, but if your's doesn't, or you are building a larger component h-bridge, then using the outputs of the h-bridge is where these resistors would be placed). So, these resistors are connected from the pins (or the outputs) to ground, and at the junction where they connect to the h-bridge, you bring out your sense wire, which, when you measure the voltage across the resistor, it varies from something really tiny, to something larger as the load on the motor shaft increases (to a maximum voltage when stalled). The resistors must be of a very small value; sub-1 ohm is best, and depending on the size of the h-bridge, they may need to be power resistors.
You could also measure the current using a hall-effect current sensor; this has the advantage of not causing the small voltage drop like you get with a resistor (which is another reason you use a very small resistor), but these sensors can get expensive for larger currents (I've found one company that has 50A hall-effect sensors that they sell for around $25.00 or so each in single quantities - which really isn't that bad of a deal). For your small motor, though, a hall-effect current sensor wouldn't cost near that much (if it cost $5.00 I'd be surprised). Such a sensor is connected in a similar manner between the outputs of the h-bridge and ground, and output a voltage signal in a similar manner.
You'd probably run this voltage into a comparator to cause the motor to shutoff in some fashion (probably with some logic circuitry or such); you could also run it into the ADC of a microcontroller, and code the controller to stop, reverse the motor, or whatever - if the uC was monitoring it continuously, you could cause it to react to the increasing current level before any damage due to stripped gears or other issues occurred, long before stall current was reached.